You don’t necessarily have to do parentheses first

Yes, you do necessarily have to do it first

What matters is that the things inside the parentheses are a group that you can’t break apart

And outside, and you must do them first. You haven't finished Brackets until you have 5(8-5)=15.

10÷2+3-2*(2+1) you can do the division first

only because you've separated that part with a plus sign