How is TPM more secure than USB?

There is no difference in bitloker operation between a tpm or without tpm but with a usb disk. It's the security that changes, the tpm is much better than only with a usb hdd without tpm.


The tpm protects the computer from tampering when the system is offline, with a tpm there is a system integrity check before booting.
With a tpm there is multi-factor authentication, for example a pin code and/or a usb hdd.

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BitLocker Drive Encryption Partitioning Requirements
BitLocker must use a system partition that is separate from the Windows partition. 


The system partition:

Must be configured as the active partition.
Must not be encrypted or used to store user files.
Must have at least 250 MB of space.
May be shared with a recovery partition.

https://docs.microsoft.com/en-us/win...iew=windows-11

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What is the recommended boot order for computers that are going to be BitLocker-protected?

You should configure the startup options of your computer to have the hard disk drive first in the boot order, 
before any other drives such as CD/DVD drives or USB drives. 

If the hard disk is not first and you typically boot from hard disk, then a boot order change may be detected 
or assumed when removable media is found during boot. 

The boot order typically affects the system measurement that is verified by BitLocker and a change in boot 
order will cause you to be prompted for your BitLocker recovery key. 

For the same reason, if you have a laptop with a docking station, ensure that the hard disk drive is first in 
the boot order both when docked and undocked.

https://docs.microsoft.com/en-us/win...quirements-faq