and I was talking about Pi-Hole

Well, you said "you" so I thought you were talking about me since you replied to my comment.

Firefox and Telegram for example has built-in DNS if I’m not wrong. (you can disable it easily)

Right. I don't know about Telegram but in Firefoxes case I think it's disabled by default. I specifically checked that on my Firefox so it won't bypass my OPNsense.

We are sharing our use cases. And my context was “I don’t understand why people even talks about Pi-Hole”

You don't see it, do you? First you talk about your use case but then you talk about other people. So not your use case anymore. In their use case a Pi-hole, AdGuard Home, NextDNS or whatever else maybe makes sense and isn't a bad choice.

EDIT: Also, I think using your phone for other things is wrong, they aren’t really designed for that, they aren’t that secure as a PC can be.

Erm... what?? Smartphones are designed for many different things. Browsing the internet is just one of many things it's made for. It's called "smartphone" for a reason.