Well, for Dungeons & Dragons 5e at least, there is Dungeons & Dragons Beyond as well an online community for Dungeons & Dragons on Amino. Plus a more general Tabletop Realms on Amino as well for that and other tabletop games. Plus a few Discord communities. All of which I've personally used.
For Pathfinder, there is the official Pazio Forums for the Pathfinder Role-playing Game, both 1st and now 2nd edition, which is an off shoot of Dungeons & Dragons 3.5. With there being the official site for the rules called the Archives of Nethys, which also include their science fiction variant of the game called Starfinder. As an alternative, there is d20PFSRD. Which has both official rules and 3rd party rules for such.
Giants of the Playground is a good overall forum to go to if your looking for tabletop games to play, but only Dungeons & Dragons and Pathfinder, but other roleplays as well. This site is also the home of the Order of the Stick webcomic, which is roleplaying related. It has admittedly been a while since I've really been as active on this sight though.
Apart from those being more immediate suggestions, other I might offer is Final Fantasy d20, an extensive fan conversion of Final Fantasy using the general base and structure Pathfinder though with a magic system much closer to the video games and their own classes.
There is World of Darkness, which has the likes of Vampire the Masquerade and Werewolves of the Apocalypse. Also Warhammer 40k's Dark Heresay and Rogue Trader.
Though if I were to offer one big suggestion, it would be Numenera, which is a science fantasy tabletop role-playing game set in the far distant future written by Monte Cook (Very famous and well known in the roleplaying community circle), which uses the Cypher rules systems.
Basically taking the concept that a significantly advanced technology is indistinguishable from magic, with the backdrop being that eight previous civilizations have risen and fallen, each of which leaving long forgotten and no longer understood technology for the "Ninth World" peoples to find and discover their uses. Where the world is basically medieval, yet with lost technology from the previous civilizations past thousands of years ago.
Where character creation follows a unique process of "I am a (Adjective) (Noun) who (Verb)s.", which could for example translate as A Charming Jack who Crafts Illusions or A Mechanical Wright who Builds Machines.
1. The first blank, the adjective in the sentence, is filled in by a character's "Descriptor", a way to describe the character's strongest characteristic.
2. The second blank, the noun of the sentence, is filled in by a character's "Type", which is a "Glaive" (A warrior type), a "Nano" (A technology adept type), or a "Jack" (As in jack-of-all-trades). Also Wright, being for of a builder technological type, and a couple of others.
3. The third blank, the verb of the sentence, is filled in by a character's "Focus", or what the character is most known for or their special talent.
But was otherwise pretty simple because you basically use just a d20 for all your rolling. Which encounters using a challenge rating that a character just needs to meet or exceed. With ways of the challenge ratting can be decreased mattering how you built your character and other conditions, sometimes even through the help of others or using an item.