If you mean creating achievement tracks different than the existing tracks, this is not possible. You can only change the text and images.

But I see custom badges all the time -_-

They may have different images or names, but the requirements are stuff are the same,

How to rename them...?

EH, is the Fandom community is just like Brainly community?!

The CC wiki is made for asking questions/bugs/issues/problems..

Yeah, it's Fandom Community. That joke just made my cringe

Some wikis might have custom achievement tracks but those were migrated from the legacy platform. On UCP, it is not possible (at least, not yet) to create/edit custom tracks whether or not the wiki is a converted wiki.

Edit: Just in case this is what you are actually talking about, there is Medals which are displayed like Achievements but is just a custom script that needs to be installed.