Recursively merge folder on macOS (native tool)

So my sister ended up in a situation where she had been maintaining two different folders on her Mac, each one containing a huge amount of files. She needed to merge folder A to folder B and all of their respective subfolders (recursively). I discovered this little tool that does exactly that - and the best part is that it's native to macOS: ditto

Just open a terminal and use it likewise:

ditto ./folder\ a ./folder\ b

Basically ditto <source> <destination>.

And that's it. You can find more on the Man Page.