That's my experience too. We self organized on previous projects, and one day I came back from a holiday to find that on the most recent project I and a collea

That's my experience too. We self organized on previous projects, and one day I came back from a holiday to find that on the most recent project I and a colleague had been promoted to tech lead for that project. It was more an official recognition of what was already de facto. I have to say though that having official recognition that you're in this role by management also helps a lot. You need buy in so everyone in the org recognizes you as someone with "technical authority".