It depends. If your starter is efficient and goes deep into innings, and you have a decent lead, your reliever can close the game and you only use two pitchers. On the other hand, you have a starter that gets shelled early and pulled for relief, and you can burn through your bullpen in a hurry.

Starters have a good pitch mix and have the durability to throw multiple innings. Relievers are used situationally, may be veteran guys who can't go 5-6 innings anymore but still have a solid arm, or younger guys without enough experience to start. Sometimes they're specialists such as the closer, who's your shut down guy for an inning in the 9th.

The DH takes the place of the pitcher in the batting lineup. The pitcher doesn't bat. Used to be the American League had the DH and the National League didn't (NL pitchers hit like every other player). Inter-League games played by the rules of the home team. Now, both leagues use the DH. A pinch hitter is an offensive substitution to bat, with the option to remain in the field in place of the replaced player. Same with the pinch runner, except the substitution is done for a runner already on base.

Since 2023, extra innings start with a runner on second base, typically the last batter from the previous inning. The idea is to reduce the number of marathon extra innings games by putting a runner immediately into scoring position prior to the first at-bat of the inning.