Your mouth does a lot of heavy lifting when singing or talking in terms of tone quality and pitch.

Learning how to shape and control your mouth, facial, and throat muscles are a large part of learning to sing well.

I'm not a trained vocalist, but I am a trained brass instrument player, and the principles are the same.

Although I recently realized that I've been treating singing as a musical instrument when I sing in the shower, and I have a ton of things I've learned on my own that I wish there was a community here to share it with. I've gone from being a meh singer to being able to sing 3, almost 4 octaves on key, and I've made progress being able to intentionally polyphonically sing.

#1 tip is practice singing as quietly as you can. Not a whisper, but like you turned your stereo down.