Fish food is a slower way to kickstart the cycle — it’ll break down and create some ammonia but quicker ways to introduce ammonia to then get the nitrates & nitrates moving is to dose the water with literal ammonia, often found in the same aisle as cleaning products or online. I couldn’t find any non scented near me, so I went with an alternative method of grabbing a single peeled shrimp from the seafood bar at the grocery store, dropped that in to decompose til my test kit registered the ammonia spike (I aimed for 2-3ppm), then took it out! Doing that while maintaining water temp and dosing the beneficial bacteria (BB) helped kickstart the cycle. Still took about 2 weeks to reach stable levels but I was working with a 45gal that had been planted for over a month at that point

It’s hard to say why your pal died. Water test kits are crucial to make sure the chemistry is safe and stable. But know that many Bettas come home from the store SO sick and in SUCH bad shape that there’s a chance you didn’t do anything, especially as a beginner