I'm not super up to date but if you're willing to look at older games with active modding communities, the Elder scrolls, Fallout, and Stalker series' stand out as having very active modding communities.

ModDB is also a great resource (was, not sure if still the best), you can figure out where the crowds are at based on levels of submission and discussion activity there.

PS Minecraft modding is an interesting example of where those commercial revenue-focused practices have bled into the modding scene. Never have I seen so much adfly, patreon, and general money-making motivation within a modding community. I doubt it's the only one that suffers from the issue, but it's notable.

The entire Arma series has a huge modding community, along with good documentation on the internals, it's one of the main selling points of the game. Their last competition spawned hundreds of entries and awarded €500,000 in prizes to content creators.

https://arma3.com/features/content-creation

Factorio has to my knowledge a pretty neat modding API internally.

Skyrim VR on PC is absurdly beautiful fully modded. The vortex mod manager along with the nexus mods community are great.

Edit: In terms of getting into programming, you can create mods with custom scripts you write to change the behavior of the game.