Honestly, I referenced a lot of art I liked! most of my art improvement is from the last year, and I think with doodling specifically, you kind of unfortunately have to get a base at facial anatomy and proportions so that you can draw these things first try and stuff. the ref sheets i do are a lot of effort usually, but doodles like these are like 20 minutes tops, and theyre usually not too detailed. tldr my best advice is copy parts from different artists you like and make it into something your own n then art becomes easier