Amazon gives you 5% back for using their credit card, it's criminal not to use it.
If you buy a lot of equipment or expensive equipment - B&H credit card covers sales tax! I.e. 10% for my area! (I don't use it since I don't buy that much, but still it's an option)
I know I could probably min-max this into more by juggling different cards for things like Amazon and Costco but I'm lazy and don't want to think.
There are fee free cards that give cash back as statement credits (AMEX Blue iirc). No limitations on what you can spend it on. The Apple Card does 2% cash back which you can just transfer to your bank account.
The Amazon card requires a Prime membership, but gives 5% back on anything bought at Amazon. I bought my last TV using the 5% back I had received.
Then there are top tier cards like the Chase Sapphire or Cap One Venture X that have yearly fees. But, if you take 1+ trips/year they immediately pay for themselves and more (credit for global entry, yearly statement credit for travel that almost equals the yearly fee, lounge entry, etc...). I routinely use points from the Venture X to cover travel expenses like tickets, rentals, hotels, eating out, etc...
https://frugalprofessor.com/bank-of-america-customized-cash-...
To your point, it's not free money at all: the credit card companies are collecting fees, and the merchants are passing them on to you. This is a way to claw a part of that back - if you don't use a rewards card, you're paying _even more_.