From my understanding, full-game runs use RTA and include loading screens, while track-specific level leaderboard runs use in-game time. Is this correct?

Also, when does timing start and end?

Thanks :)

Yep, full game runs are primarily RTA (races, loading, menus, etc are all timed), but you can add your sum of race times as IGT if you want. ILs use the time you're given at the end of the race/lap. Timing for RTA starts when you leave the main menu and start loading the first race

RTA runs end when crossing the finish line on the last race, right?

Yep, as soon as you cross the line you can stop time

I did a 3 lap short season and I started timing after clicking continue in the race settings, is that okay?

Timing starts when you begin your first race, so you'll want to start the timer when you hit continue and line up on the grid

Thanks for the information, will be running this shortly!