i think a lot of it for me comes down to having good references. if i have an idea in mind, i usually try and find (or take) photos that are as similar as possible to what i'm trying to achieve. almost all of my paintings are done with the default procreate round brush, though i also use the "spectra" brush if i need to add a bit of texture, or reduce the harshness of the lines. the rendering itself mostly comes down to making use of the pen pressure, and adjusting the colours slightly as i go.

practice of course helps a lot too, as does watching to see how other people render lighting. if you're interested in getting examples, james gurney has a lot of really great videos on this!
https://www.youtube.com/@JamesGurney/videos