Are my ablution and prayer valid if I find a piece of food in my mouth after rinsing and remove it, or during prayer and remove it? And what is the criterion for food in the mouth that does not invalidate the ablution?

It is clear that you have many obsessive thoughts, so we advise you to disregard them. The presence of a small amount of food does not affect the validity of ablution (wudu') or prayer (salat). What usually remains in the mouth from food does not prevent water from reaching during rinsing (madmada), because water is subtle and usually reaches every place. And if it is assumed that a large amount of food remains that prevents water from reaching, then ablution is still valid, for rinsing is a sunnah (definition) according to the majority of scholars.