Steam
Epic
GOG
Origin
and quite a few others...
The days of buying game on disk at a brick and mortar store are gone, it's the digital age now
There's also Ubisoft's Connect, Microsoft's Game Pass and EA has their own platforms but they are not as good as the platforms John mentioned.
Steam is the most widely used game store platform in the world but Epic and GOG are awesome too.
What I like about Epic is that they offer a new game for free every week, sometimes they are junk but other times you can hit the jackpot and get really cool payware games for free. Also once or twice a year they will give you a 20% off coupon that even applies to games already on sale. I've gotten some amazing games for dirt cheap because of their deals. They also have a lot of unique Indie games there.
What I like about GOG is that is has many older games that you cannot find anywhere else, all their games are DRM free and GOG also sometimes modifies old games to make them more compatible with newer OS's and with widescreen monitors, etc.
Also I should mention all three platforms, Steam, Epic and GOG offer some really cool free games as well.