Is there a way to create a macOS smart folder which uses an "or" search? E.g. find all files which have tags "A" or "B"? It seems that smart folders allow only to search using an "and" command.
Yes there is.
If you press the OPTION button while composing the query in the finder window, the "+" button changes into a "..." button, that exposes the interface that allows for testing "All | Any | None" of the subsequent conditions.