I love this stuff. I’ll dip my toes into biology, but my real love is culture, which is shaped by and related to so many other things. For me, the process of developing a species goes back and forth between ideas and questions. First, I’ll come up with some sort of premise or triggering idea. (e.g. “what if I had a species with blue skin?”) This will lead to some questions (“why is their skin blue?”), and answering those question leads to new ideas (“their blood is blue”), which in turn lead to even more questions (“what does having blue skin imply about their biology?”).
Sometimes the ideas are big ones, so the questions narrow things down. (e.g. if they’re ruled by a monarch, what are the powers of the monarch? what’s the monarch’s name? where does the monarch live?) Other times, the ideas are small, so the questions lead me to think about the broader picture. (e.g. if they’re ruled by Queen Angela, is a monarch their normal form of government? Is there some kind of parliament/nobility that Queen Angela has to deal with? Are female rulers normal or unusual?)
For actually mapping stuff out, I usually use Google Drive or OneNote to write down everything I know about the culture up front. (e.g. where they live, their magic system, any interesting things I’ve already decided about them, etc.) (actually this is often on paper first, but I transfer it to a digital archive sooner or later) Then I go through and flesh out whatever area catches my eye, keeping in mind that a decision on one part of a species or culture will impact other parts. (e.g. a decision about the military could impact how they interact with their neighbors, what kind of government they have, etc.)
I’m also big into conlanging, buuuut that’s a slightly detached hobby. I do incorporate conlangs into my worlds, but I’m a big believer in as little conlang as possible in an actual published work, so I don’t focus on it as a big part of worldbuilding. (besides, you can knock out a passable naming language in a half-hour)