Then for a day and a half after I was working on that spreadsheet, it showed up at the top of the suggested videos.
Again, which applications had access to your clipboard and user files at that time? If any of the applications running on your computer was stealing your data and selling it for financial gain, Google would likely be buying it and obviously using it against you.
You also have to consider side-channels. Were you or your friends talking about that spreadsheet project via Discord or some other known abuser? Did you talk about it with a person in your room while daddy Google or Amazon were listening? (Alexa in the room, Google assistant on your phone etc.)
in short: years of nothing, nothing, nothing, TWO DAYS OF TRANS VIDEO SUGGESTIONS, and then since, nothing, nothing, nothing.
This might simply be expectation bias. You may have been shown such suggestions in the same pattern before and simply didn't notice because, contrary to the present, the topic wasn't on your mind and simply forgot about it because you're being shown irrelevant suggested topics all the time.
Even after reading a lot of people telling me that it is just The Algo^TM^ at work, that incident seems so razor specific to activity I was simply doing on my computer at the same time Youtube was open rather than anything that could be related to my personal interests.
That's how "The Algo^TM^" works. Google gathers data on you directly through its applications, from 3rd parties selling data they stole from you and indirectly through the same process from people you associate with.
It's even possible that some data broker simply made up the fact that you're trans. Google could have then assumed it's true because you associate with trans people here. I could very well see that happen in an enshittified system such as Google.