Like, If I start the video, create the world, play, exit the world, end the video. Then the next day, start another video, open up the same world, show no changes in the settings, and play again, would it still count?

Depends on the game, sorta. Almost every game requires that the run be done in a single segment. The only exception I can think of are specific categories for segmented runs, but those are rare. Short answer, yes, do your runs in one sitting.

I do know that in games like minecraft or karlson, pauses don't count towards the final time, but pausing for days is just cheap and will likely result in a rejection. I think that celeste has a specific limit for how long your breaks between segments can be