If you want the SUPER technical anatomical definition. Then the arm's beginning is more vague since you should take into account muscular, skeletal, and vascular/nervous anatomy.
Skeletally, the arm begins from the humerus. However, you should also take into account that the shoulder is made up of the connection between the humerus, scapula, and clavicle. Thoigh not specifically part of the arm, both the scapula and clavicle are still included in the appendage.
If you're talking about muscles, then literally every muscle connected to the humerus is considered part of the arm. This gets a bit weird since it means that pectoral muscles (chest muscles) and some back muscles are part of the arm.
Lastly, the most concrete definition of the beginning of the arm comes from blood vessels and nerves since the brachial artery (arm artery) is clearly defined to begin as a continuation of the axillary artery which ends after passing a muscle called teres major. This essentially separates the arm from the armpit region with a clear border.
In conclusion, I probably wasted your time using uncommon anatomical terms so here's a TLDR: The arm kinda starts from the shoulder but a little forwards and kinda backwards, maybe? I don't know! All I know is that if a doctor tells you they are inspecting your arm, then they will check for far more than you'd think.