I try to explain things as good as I can, though I may confuse some things between editions. I will stick to tieflings, else I have to talk about the whole multiverse. Big thanks to Niara.
- Tieflings are planetouched from the lower planes. The inhabitants of the lower planes are fiends. There are lawful evil devils, chaotic evil demons (both fight each other in the blood war) and there are many other fiends who are neither devil or demon. Tieflings can be influenced by any of those. Planetouched does not mean they are children of fiends, it only means they or their ancestors were influenced by the lower planes. This does not determine their alignment.
- Given that tieflings can be influenced by any fiend, I think they look too homogeneous in BG3. But the recent events make it plausible that many of the recent tieflings are influenced by devils. At the moment you can only select different devil tieflings as player, unlike the Pathfinder game were you can select many backgrounds (devil, demon and others).
- Unlike tieflings, cambions are really the children of fiends (any kind of fiends) and mortals (any kind of mortal). They are usually evil and have magic powers similar to full fiends. Raphael is likely a half devil because of his obsession with contracts.

Prof. Dr. Dr. Mad S. Tist

World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already