What stops the hardware wallet manufacturer from stealing my cryptocurrencies?
What stops the manufacturer of your hardware from using a backdoor and simply stealing your cryptocurrencies? How much do you have to trust hardware wallet manufacturers?
While a completely “trustless” solution is probably not possible, manufacturers are doing everything they can to minimize the need to trust them.
Most of the software code of many hardware waller manufacturers is open source, i.e. publicly available. Anyone can check how the device works and how secrets are handled. Of course, not everyone has the ability to review code: that’s why independent researchers are often encouraged to analyze, and are often rewarded by bug bounty programs when they find something. This does not limit their ability to publish a full independent report without permission.
The essentials to go: A wallet manages your secret private keys and requires full access to them. You can and should demand full transparency about how a wallet works and ensure that independent public audits are encouraged.