Well, after singing for a while with a teacher I kinda understood how to produce a good steady voice. However, it's a bit difficult to explain in text.
But okay, this at least was the revelation for me: try to blow air like blowing out a candle. Meaning a directed, focused blowing that comes with a good strength. This is basically how you want your breathing to be when singing. A steady air flow, without muscular tensions. Then, you just slowly add voice to that air flow. With a humming voice first, trying to utilize as much of your vocal cords as possible. Then you can try singing louder, and seeing if you can keep the same steadiness. What you are trying to get the hang of, is a certain "bite" where your voice resonates with a good clarity and has strength to it, without really trying too hard. (Go watch John Denver singing to know what I mean)
An issue I, and probably many others, face is an inefficient breathing that generates friction in your vocal cords, causing your voice to quickly fall apart becoming weak and crackly.
So avoid "pushing" your voice as much as necessary. It will just cause bad muscle memory, like doing deadlifts with a bad technique. You have to build up the nervous system and the muscle memory slowly, never extending it too far. It might feel annoying singing with "half-volume", when all you want to do is to sing along with your favourite singer with full power. Yet once you build up the technique the whole thing becomes significantly easier. Hard to explain, but you shouldn't be feeling like you have to expend a lot of effort to get a strong voice.
Well, I hope that's something. I would be curious to know if it helped.