The small circle cover next to my door fell off. What is this part called? is it expensive? Is it easy to replace by myself? I circled the part I'm referring to in blue.

 

that's a plastic cover for the bolt that hold that corner piece in place. You can pry it out carefully to reveal the bolt. The purpose if that entire piece it to protect the metal in that corner from rock chips that can easily damage the paint and lead to rust eventually it seems.