How do banks determine interest rates for high yield savings accounts?

Seems like the most competitive high yield savings offerings have dropped from ~5% a few months ago to ~4.5% today. What causes this / what will cause them to go back up?

I see T-Bills seem to also be around 4.5%; I assume it’s all connected.

My bank (Synchrony) dropped down to 4.3% so I’m thinking of moving everything to Brio or Varo who apparently still offer 5%.