What is the difference between בפנים and בתוך?
Generally בפנים is used to mean "inside" like "indoors" and בתוך is like "inside" or "inside of."
Why does בפנים have the plural ending?
It doesn't. It's an adverb - they only have one form.
beefnim o bakhutz
What is the difference between מחוץ/ בחוץ?
בחוץ is outside.
מחוץ ל... is outside of , like outside the house, outside the restaurant, etc.