You typically want a mix of UDP and TCP (or sometimes a weird TCP-like monstrosity rebuilt on top of UDP).

Taking great care to design your data streams to be self-correcting (e.g. transfer a world location where characters are going towards instead of which direction they are heading) can go a long way towards saving a ton of synching headaches, and enables very efficient networking.

One-off events go over the TCP-like channel, but constant streaming data that naturally self-correct over time, like the example above, can benefit from being on the UDP channel.

If some packet loss is not problematic, UDP is used. Often for players positions.

That's also how some speed hacks are made possible.