I think to simplify things it’s worth knowing that the only partnership at the moment where you can transfer actual points from one frequent flyer program to another is Krisflyer and Velocity.
As far as your Amex reward points go, you can transfer those into various other reward programs up to 27 airline and some hotel rewards programs depending on your particular credit card. In order to do that you must become a member of that frequent flyer program eg. Krisflyer, Etihad Guest etc.
Once transferred you can then use that particular airline’s FF program to book reward flights. In the sample you gave " I could transfer them to Etihad them make a booking using Garuda given that it is a partner airline" because Garuda is a partner you should be able to book Garuda flights through Etihad. In that case you need to phone the Etihad Guest Service centre to make the booking as they will not necessarily show as reward or redemption flights online.
Hope that helps.