Keyboard playground
Here you can see the text field that lets you to test Russian-Tatar united keyboard layout. It doesn't matter what layout is set in your system — in this field only Russian-Tatar one will be used (there is an on-screen keyboard replacement for mobile devices). As a result you can get your hands on the layout to find out how it works.
alt gr + а (or option + а)
-> "ә"
alt gr + ж (or option + ж)
-> "җ"
etc.