Philosophers aren’t just preoccupied with fundamental truths - they think a fair amount about what truth is.
Most of this summary is lifted from Blackburn’s recent book “On Truth.” It’s fantastic. You should read it.
Correspondence Theory, SEP - https://plato.stanford.edu/entries/truth-correspondence/
Coherence Theory, SEP - https://plato.stanford.edu/entries/truth-coherence/
David Hume - Why You’re Probably Wrong about Everything You Know - https://medium.com/personal-growth/david-hume-why-youre-probably-wrong-about-everything-you-know-62fb4caa2b21
IEP on Hume - https://www.iep.utm.edu/hume/