> Is it common for closed source software to give the source code to customers to build?

Yes, somewhat common. I work in embedded, and where a supplier gives us the ability to build their code it is a lot easier for us to deal with the next time we need a new build from them.

Closed source software could apply too, if the execution model supports it (e.g. WebAssembly). Just because the format is binary and "unreadable", its permissions (accessible functions) don't have to be