If I format to ext4 a USB and store root owned things and other things and lose like my main user that I used to own and write it, can I use root with new user like sudo to get at those root owned things? It would be root owner and group things on that USB maybe or maybe not, maybe.

Yes.

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Note that the ownership is stored in terms of UID and GID numbers, not the actual names of the user and group. UID 0 is reserved for root, so that'll be the same.

But for any normal users, you can't count on the UID for a particular user name to be the same. If you're using something like Ubuntu or Debian, than the first normal user created will be UID 1000, but that's just convention not a true "standard". So you may be surprised to see files/directories owned by a user name you don't expect.

For example, suppose you mount your USB drive on an Ubuntu system where the first normal user created was "jos". Then all files with UID 1000 ownership will show up as owned by "jos". But if you unmount this USB drive and then mount it on an Ubuntu system where the first normal user created was "may", then those same files will show up as owned by "may".

Personally, I always set the first user to the name "kuo", which has saved me some confusion but of course I can't count on anyone else's computers to follow my convention.

If a file/directory is owned by a UID/GID that doesn't exist in the OS you've booted up (or if you deleted that user or group), then the file/directory will still show up - it just won't have a friendly name to show instead of the UID/GID number.

Since "root" will always be UID 0, those will always show up as expected.