Wood frame compose

We usually use pine for door frames I think is the cheapest wood to work because is gonna be painted But you can always use any kind of wood . It depends on what kind of wood is your door. .I mean when it is gonna be varnished.

It depends on what you want and how long you want it to last. Cheaper wood's pine etc are fine but if you want quality go for a hardwood. Obviously the hardwood is more expensive.

you can use a Hard Pine which is more durable and less likely to warp or move or shrink. a bit more expensive but worth it in the long run

Yes generally speaking the choice is Oak or Softwood, generally speaking i prefer the frame & archs to the skirting.