Hmm, poorly.

I used to keep paper lists, then I marked several thousand books on Goodreads - which got ported over to StoryGraph when I switched.

Currently, I take screen shots of stuff that looks interesting and look them up at the library. I can save them to a list function if they exist in that particular library system (I have a couple library cards but I prefer print copies vs Libby). One of the libraries lets me sort the TBR list by "available in x branch now" for when I'm in branch picking up holds. The other does not, so sometimes I just email myself books that are in my closest branches if I think I'm going to do a pickup soon.

If none of the libraries have it I make a recommendation to the library to purchase if it fits the guidelines (published recently), or save to a online second-hand book selling site if not (better world books mostly, but sometimes I get Amazon gift cards, and chapters/indigo if I want a fresh copy for a gift or something.)

I tend to have very specific reading obsessions for a month or two so I'll read what the library has and then move on to a different topic vs planning out a real TBR. If it's a reoccurring interest I can tag/organize the library lists for when it comes up again.