Google Fi is a simple, low-cost cellphone plan from Google in the U.S. I don't use much cellular data as I make heaviest use of the phone at home, where I can use wifi, so paying per-GB is much cheaper for me than any of AT&T's plans. I was paying about $60 a month with AT&T, but am now around $25 (in Feb 2022). I really like that cellular data costs the same in many foreign countries and tethering and wifi hotspots are included as standard. I've used the plan in a couple of foreign countries now, and it works great though it takes my phone 10 to 15 minutes on arriving for roaming to become fully functional.
Google Fi has deals with various cellphone networks, and some modern phones are capable of switching between networks automatically as you travel around the U.S. I find that my phone spends most of its time on T-Mobile's network, but that might just be because it's the strongest of Google's network partners where I live. A couple of times I've noticed my phone saying it has no cellular signal at home, a problem I never had with AT&T. Apart from that, I've not noticed many other differences compared to AT&T.
Cellphone companies have traditionally assumed you'll buy a new phone every couple of years and allow you to periodically purchase a heavily discounted phone at the expense of a more costly plan. Google Fi doesn't offer periodic discounts, benefiting you if you tend to use the same phone for more than two years. If you buy a phone in the Google Fi store, you can choose to spread the payments over two years at no extra charge.
If you think Google Fi may be a good fit for you, see the Google Fi website.