What is Someone titan *#21# and what does it do?

If the question is just what is *#21#, it just is the code to see if your own number is set to forward the call to another number (eg the GSM standard code *21*phone-number# will turn on call forwarding to phone-number). It just tells you what settings -> phone -> call forwarding shows you. Dialing ##21# cancels call forwarding on your device (or again, just turn it off in settings).

What confused me was the what is “someone titan *#21#” as that made no sense to me when I first read it.

It’s easy enough to find any of those dial codes are - Wikipedia lists them, CNET, a dozen other tech sites, most carriers have them buried in a support document somewhere. It takes a 2 second web search to find out what any one of the dozens of dial codes are.

Without any context at all, looks like nonsense to me. You need to give some context - where did you see this, what were you doing when you saw it, was there anything else other than just that?

*#21# was put up on tiktok as some sort of iphone diagnostic code.

I would ignore it as fake news

Actually, the codes themselves are pretty standardized worldwide under the GSM association, but not all carriers support all codes. If the code isn’t support you just get a message to that affect when you try it.

I believe it has something to do with call forwarding. some people think that it's a means to detect being spied on

https://www.techjunkie.com/tell-call-forwarded/

and they vary with providers and countries.