A lot of things factor into it and mainly it’s all personal preference.
I draw zoomed in at about 200% on a 1920x1080 canvas so when i zoom out afterwards the line quality looks sharper and smooth.
There’s also stability that i have set to 6 for myself which also determines how smooth the lines are as well as how much delay your tablet pen has. Which in some cases for lining i have set to S-3 so i can make easier curved lines.
I also use the rotation and zoom tools a lot, and i erase and fix up any messy pixels.
If you want “straight” lines, shift and mouse click from point a to point b.
And lastly, don’t use actual black for your lines cos it always looks mega messy lining with that, i recommend a really dark grey but never 100% black.
As i said, these are all personal preference stuff as well as just being familiar with the program and slowly practising and improving my style so yeah, i hope this helps!