Quoting from a similar thread from 2015:
It started with the good old THE HARDEST VIDEO GAME BOSS EVER (https://www.youtube.com/watch?v=sQZuidKexBQ), and I'm pretty sure the related videos took me to Dracil's IN Extra clear (https://www.youtube.com/watch?v=4Nb5Ohbt1Sg). At the time I thought these were all arcade-only games, so I left them alone for a long time.
In 2012, I saw a friend playing IN, and I recognized it from the videos I had seen years ago. The rest is history documented in my signature below haha
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In addition to the games, a lot of my early Touhou experience was finding and watching way too many fan videos on Youtube. Lots of MMD trash, but also quality videos like Touhou 3D Dog Fight (https://www.youtube.com/watch?v=sLTpJJAraG8) and Burning Meiling (https://www.youtube.com/watch?v=UbTgBxmXVds).
Also discovered the music circles Demetori (https://www.youtube.com/watch?v=8EdW28B-In4) and CC Style (https://www.youtube.com/watch?v=0IqvDWJB8Zo) around this time. They make good stuff.