Are master bulds unique and non changable?

They are unique yes. Don't understand what you mean by non changeable.
Some builds may have an equivalent binary, as some PRs may alter other things rather than the main emulator's code (e.g. some readme, unrelated scripts, etc).

Ah , gotta learn to write better ! thought unchangeable like : build 6661 will be absolutely identical when downloaded now and in 2 months for example