How to stream on Twitch? Steps to start on PC/smartphone and recommended OBS settings

I want to try streaming on Twitch. But I keep seeing terms like 'stream key' and 'OBS,' and I don't know where to start.When you look it up, you'll find methods for streaming from a PC, a smartphone, ...

Deciding before buying a PC. Smartphone, tablet, or PC—which way of starting streaming suits me?

I want to start streaming, but is it impossible without a PC...?”When you search with that thought in mind, gaming PCs, ...