I keep a checking account open with a local bank just for the brick and mortar perks, but I use Fidelity for any savings and investing outside of workplace retirement plans. I tried Vanguard, Schwabb, and a number of other financial outfits, and Fidelity came away as the easiest to use and the best support (I always get an agent based here in the U.S., never some outsourced person like so many other companies are doing).
At Fidelity, you can open a CMA (cash management account) and it effectively acts like a checking account, except you can use any cash to buy holdings like SGOV or VUSB, which are ultra short-term t bill funds that offer the same or better rates than many of the products listed so far, and they aren’t subject to longer term rate lock ins. Your money is never more than 2 days away, since you need to sell the funds and let the money settle before withdrawing, so you’re still very liquid. You could also just leave the money in SPAXX too as a default.
In my overall investment portfolio, we’ve got a solid sized cash wedge so we don’t have to sell when the market dips, or to deploy cash at good opportunities. Keeping that cash in the manner described above keeps it from losing its luster to inflation (as much as possible) with minimal risk of the investment going down. If you’re in a state with state taxes, it can be tax-free income as well (still taxable for Fed).
* Not investment advice