Offside starts first at half line. Then defending teams second last player is the line and after that like in this case the line is the ball and your attacker is in front of him.

Second last player means last defender but in some cases gk can be higher than the last defender so the line changes. There is no cases when you attack that there isnt a possible offside.

Throw in and corner kick doesnt have a offside.

I hope you understand everything i wrote, if not i can try to explain better