It's all encrypted in storage. The decryption key is in the secure element / TPM chip, additionally protected by your PIN / password. Shutting it down unloads all encryption keys from memory.

Beware that US customs / immigration / border control can seize your phone and refuse entry.

Yes, but customs can still compel you to unlock your phone as we have recently seen with the Norweigan tourist who was denied entry due to having a JD Vance meme on his phone.

I would recommend having a separate phone with non-important data on it to take with you to the US, or have a self hosted cloud service that you can backup your data to before wiping your device.

You essentially don't have rights at the border (or in general with the current US government).