Pardon the maybe dumb question.

Do actors always get paid based on whatever they agreed to when signing on or do they ever get paid more based upon how well the movie does at the box office? For example, if an actor made a modest amount on a movie that ended up killing it at the box office do they ever make anything off of that or is that all just to the studio & execs?

Depends entirely on their contract, and typically only A tier actors can get gross points on their contracts.

For example, Harrison Ford and Jamie Lee Curtis both made serious money on Force Awakens and Halloween (2018) as they were the A list veteran actors and the movies hinged on their showing up.

Whereas back in the day, JLC made like $8k for appearing in Halloween as a newcomer, and made nothing off the movie's success.

Or for another example, Mark Hamill made over $1M for like 30 seconds of screentime in TFA, which was due to his star power.

He didn't make much from the entire OT, certainly not millions.

RDJ made something like 50m off of endgame because it was based on box office