SPAXX is a money market fund, and it pays dividends, not interest. That’s a bit of a technicality, but there are some important distinctions in how they work.
Money market funds don’t pay a fixed dividend, they just pay based on whatever the fund earned for the day, and you get the dividend once/month (usually the first day of the following month). So treat them like a mutual fund that never changes from $1/share and pays dividends monthly. The important number to look at imo is the 7-day average, which says how much it has returned over the previous 7 days, and currently that is 5%. If you look it up on Fidelity, you can also see returns for different periods, just like with mutual funds.
What’s the min amt to can buy at?
For t-bills, you have to buy in $1,000 increments, and you can only auto-roll if you buy new issues. Depending on the maturity of the t-bill, new bonds are issued weekly or every four weeks. Here’s the tentative schedule for various durations of t-bills (PDF warning).
So if you want a 6-month t-bill, you’d buy in $1k increments evenly distributed over the six months. I personally do 13-weeks (~3 months) because the other half of my efund is in ibonds (will change soon), and I bought every two weeks because that seemed the easiest. If I went for 26 week t-bills, I’d probably buy every 4 weeks or so. You’ll probably want to space them out instead of buying all at once so you can just cancel autoroll instead of selling on the market in case t-bill returns increase dramatically and your t-bills are worth less (e.g. the thing that killed SVB).
If you’re lazy, you can just buy a t-bill fund like TBIL if you just want to dump money in instead of building a ladder. I personally like the ladder though, so I went through the effort to build it.
All of this applies to Schwab as well, and they have better money market funds (higher returns), so you may want to consider going there. I picked Fidelity because I already have an account there and won’t be moving anytime soon because their HSA is unmatched, but since you have both, you should look at what Schwab offers as well.