Open source software is made freely and available for possible modification and redistribution by anyone.
Bitcoin is open source and I think that Hardware wallets should also be Open Source so that everyone can check and verify their code.
Not all wallets are open source, and who knows what code they have and if someone can post firmware update with big bug or malicious code.

Open Source Hardware wallets

• Trezor
• Cobo Vault
• ColdCard
• BitHD*
• Secalot
• Bitbox
• Passport**

*using Trezor code
**using ColdCard code

Open Source can also have bugs, and code should be verified and checked before installing.

If anyone knows any other open source hardware wallet please post it here so I can update my list and make corrections.