I've looked everywhere but I can't find anything. People keep saying DSDA but I can't find a link to the main levels. Can someone help?

DSDA is for source port Doom. Steam version, you're literally here. But, on the category extension leaderboard. There are tabs below the game's name that show you main leaderboards, levels, news, etc.

Also, if you are launching the game from within Steam, you'll get a pop-up asking which version of the game you want to play. For these leaderboards you are at, we run the Unity version. Which is the very first option. Everything else on that list there is no leaderboard for, as they are just dosbox ports that nobody cares about.