Multiple Listing Service (MLS) Details
The Multiple Listing Service (MLS) is the main way that agents input real estate properties onto the market for sale, and in many cases, for rent. There are different MLS around the country. Some states have only 1, while states like VA have more than 1. Sometimes multiple MLS will intersect to a high degree, with no 1 MLS having more than 50% of the properties in a given city/county (i.e. Williamsburg/James City County), while in other cities/counties, 1 MLS will cover >99.5% of the listings (i.e. Norfolk). Realtors and real estate firms need to pay for access to each MLS. Even when an agent has access to more than 1 MLS, they won't always put their listings in each MLS that they have access to due to the time-consuming nature of MLS entry and paperwork. From the MLS, especially when accompanied by paid programs that companies like Garrett Realty Partners use, the listing auto-populates to 100's of websites where buyers can search for listings.
Here is an MLS map of the US:

Image courtesy National Association of Realtors
MLS in SE Virginia
Here is a close-up of the MLS in Virginia:

Image courtesy National Association of Realtors
Among those MLS, here are the 5 ones that I have access to:

Image courtesy National Association of Realtors
While it's unusual for a real estate agent to have access to 5 MLS, I find it very helpful since I cover a large territory , since I am close in proximity to so many MLS, & want to provide the best services that I can to the buyers and sellers I represent. If I only helped buyers and sellers within the confines of Washington DC, on the other hand, I might only have 1 MLS (BRIGHT), since it's so distant from other MLS on all sides (>80 miles).
Within SE VA, excluding the Eastern Shore, here is a breakdown of the most dominant MLS by city/county according to my research:
Real Estate Information Network (REIN) MLS based out of Virginia Beach:
(still working on this next one as of 11/29/22)
Central Virginia Regional (CVR) MLS based out of Richmond & Chesapeake Bay & Rivers Association of REALTORS® (CBRAR) MLS:
I group CVR & CBRAR together because, unlike other MLS in SE VA, they have a co-op. The co-op means that any agent with access to 1 can pay an additional amount per year which allows for a single listing data input form to be used instead of 2 (saving tremendous time) & allows for a single search for buyers to cover both MLS.
Northern Neck Association of Realtors/MLS (N Neck):
(still working on this next one as of 11/29/22)
How to Find Out Which MLS are Used Most
Whether you're an agent or a buyer, it's best to check on Zillow for which MLS tends to be used from active listings (not sales, since only 1 MLS is placed on Zillow for sales even if the listing was posted on more than 1 MLS). Sometimes that's easier than other times. For instance, in this screenshot of Emporia sales, you notice that only 1 of the properties lists which MLS is being used without you clicking on that property.
In some cases these are displayed prominently, as in these cases where you see a picture of the MLS logo on the bottom right of each picture of the desktop view on Zillow:

Image courtesy Zillow.com
In many cases in SE VA, they aren't displayed so prominently:

Image courtesy Zillow.com
Those can also be present in some cases on the list view from the mobile app:

Image courtesy Zillow.com
You can toggle between list view and map view on the top left:

Image courtesy Zillow.com
Once you've selected a property, you can scroll down in the information until you see the MLS providing the feed to Zillow. Here's that view from the app:

Here's that view once scrolled down on a property from the desktop version in the overview section:

Just like you can't tell if a property is secondarily listed via the co-op (CVR & CBRAR MLS) on an MLS search, you can't tell on Zillow either.
If you're an agent, the method I used where you check MLS for sold listings, is a solid one for those MLS to quickly acquire a high volume data set that will be very accurate at least for those MLS that you have access to. That said, it's best for agents with more than 1 MLS, with the higher the number of MLS you have making the numbers more and more accurate. For instance, my Williamsburg & James City County #s are likely the most accurate because they have other MLS in between them and MLS that I don't have access to on all sides. On the other hand, you'll notice that on my spreadsheet there are some counties where I had painstakingly go through Zillow on a number of listings to get relatively accurate percentages.
Keep in mind that a single city/county could have 1 MLS having the majority of properties in one section and another MLS having the majority of properties in another section, especially when adjacent to state borders & a large county.
Why Important for Sellers
The higher the exposure of a property in marketing, the more action a home will tend to get with showings, the higher the number of offers, and the higher a home will sell for statistically. Did you know that 29% of buyers found the home they purchased through their agent (NAR)? Because of that, which MLS your home is going to be listed on is a critical question for sellers looking to list the property.
That's especially true in areas of high crossover where a single MLS isn't dominant. In my research, neither Gloucester County nor James City County/Williamsburg had more than 51% of listing sales in a single MLS. When I interviewed a former agent and won the listing, one of the top reasons why he stated he chose me was due to me being in more than 1 MLS. The higher end of the home, the more MLS it will tend to be on as part of a superior marketing package more common to experienced agents than to new agents since most new agents only start with 1 MLS & many agents remain with only 1 throughout their career.
Example:
I've seen where a buyer's agent based out of Richmond reached out to me for a buyer he was representing looking in Hampton where it was obvious that the agent had no idea about the competition in the neighborhood because he was looking on Richmond MLS, which I was advertising in, but where other agents weren't due to the distance from Richmond (about a 1 hr drive) & the low percentage of properties in Hampton that were listed on Richmond MLS (<2.5% last I checked).
Why Important for Buyers
Many buyers are unaware of how much better a search feed on MLS can be vs. a search on public websites common to buyers like Zillow. How much better depends on the MLS, with larger MLS typically having better search features. Unsurprisingly, the least user-friendly MLS I have access to is also the one with the lowest number of members. By establishing a feed with a buyer's agent, you can weed out many of the properties that you wouldn't be able to on public websites, saving you tremendous time and hassle from not needing to look through a number of properties that you wouldn't want. A small number of buyer's agents, myself included, have spent hours on search templates that they can reproduce with a few clicks to start as a baseline for the modifications on your search.
Here's an example of a search baseline I created for USDA buyers where I recreated the USDA map:

Image courtesy Rein.com
I do recommend using both MLS & a search on public websites like Zillow in certain cases (for instance, if you are working with an agent who doesn't have access to the MLS