Does creating an installation USB from here and then using it to reinstall the OS allow full access to all files on disk including the previous Windows OS files? Or is there a way Windows will prevent access with out-of-the box settings somehow?
There is no defense against a knowledgeable person that has a physical access to your computer. There are so many ways to access your data by booting a malicious USB.
Your only possible defense is to either disable USB in the BIOS and place a BIOS password, or to install a physical padlock on the computer. None of which is a pretty good solution.
If there is some particular data that you would like to keep safe, encryption is the best method. See for example VeraCrypt. Bitlocker was also mentioned in a comment.
Be very careful when encrypting to properly save the encryption and recovery keys, to be sure of getting to your data in case of a catastrophic failure.