Entirely dependent on the game and community rules. Check the rules, forum page, or ask a mod.

You can use Emulators most of the time, however only specific ones (or specific versions) because of accuracy. Also not every leaderboard allows runs done on emulators.

Below you can find an old topic about accurate emulators (pretty sure people still use most of these nowadays) https://www.speedrun.com/Speedrunning/thread/19h43