Apt

apt is the software manager that minux uses, it downloads files from the real internet.

here you can find more information on how apt works and how to run a repository yourself.

Software compatibility - extra instructions you can send trough apt on how to find and (un)install your software

adding a repository - how apt knows a server is a repository

listing available software - how apt knows what software packages it has

installing/updating/removing software - how apt knows what files to download and where