Some Huawei phones are advertised with Android (open source). What does that mean exactly?

Some Huawei phones are advertised as "Android (open source)". What does that mean exactly?

Open source means that the source code is published and can be used free of charge for your own projects.

Yes, I know open source. But what is the difference to the standard Android?

Android is actually open source anyway, the other providers also use the system.

Presumably the original from Google is open source, the adapted versions from the other manufacturers are no longer.