As my answer always is when I am asked this, if you’re asking someone else what you should think about true names, you’re not going to find anything worth finding.
And also like the idea of a “true” name is inherently poisoned by fantasy novels. The most basic and boring answer really is just what name you would answer to and consider your name. Like if my name were “Billy” then for magical purposes my true name is “Billy”.
The reason it was/is a big deal to not use your true name is based around lore and belief that something which knows your name has power over you–power to summon, to bind, to enchant, to curse, to call down upon. You can use a name for a taglock, a poppet, or a focus; if you’re working with spirits, knowing something’s name, of course, is how you summon it. If you’re working with fae in particular, by giving them your name, you give them your self and identity, turning you into nothing but a hollow shell to be filled up with orders.
But the reality is that true names are in practice probably just significantly more boring than anyone ever wants to think about, but they are a very valuable thought experiment to encourage you to examine yourself and who and what you are.