So, how do we verify that the hardware doesn't generate predictable entropy, regardless of whether the wallet developers have bad intentions or not? I don't care about the OS, let's assume you use an open-source one; my focus is on those who build the hardware that is used to generate randomness.
Last year I wrote a topic about
seed generation in hardware wallets and you can find some useful information there, but true randomness is a problem for software wallets also.
I think that software wallets are even worse than hardware wallets and it doesn't really matter much if they are open source or not.
Hardware wallets are mostly using combination of several elements mixing entropy of computer and device, and some are using closed source secure element for that.
Only verifiable seed generation for hardware wallets I was able to find was for Passport, Kestone, Coldcard, Trezor and BitBox02 walllet, others are more like a black box.
One true way to be sure that seed phrase is truly random is by using properly tested casino dices, so you don't have to trust any chips or computers that create fake random results that can be reproduced.