Simplest answer: it enables programmers on any variation of an OS to get code. If I’ve got a python requirements.txt, getting those requirements installed is the same on any system I’ve got an appropriate python version. The designers of the languages effectively deemed that an important goal and explicitly put in the effort to include package management, in at least most of your listed cases.

My ISP charges a good chunk for static IP (because they consider it a business connection). However, my $10/mo VPS (which I already had for other projects) has its own static IP and I can easily proxy individual requests through it as needed. Have looked at VPS instances for $5/months before.

Depending on the service, your ISP, and how easily you can change IPs in the registration, as others have mentioned, you could just give them your public. I’ll say my own public does change at least twice a month, so if I can’t automate updating a registration, it goes through VPS or it’s a service I just skip.

They don’t have an authentication code; just an identification code. You log in elsewhere and put in the code so Microsoft knows which device you actually want to log in on. Think e.g. logging into most streaming services on a Smart TV - get the code and take it to your phone, where you actually log in.