How do I access files that are owned by another user?
Even if you have the same username on a different instance of macOS (it could be the same computer), the OS doesn't see the two as being the same.
What you need to do is take ownership of the files.
Using Finder
You can do this in finder by using &quot;Get Info&quot; when selecting a file or folder.  In the example below, I'm selecting an entire folder to take ownership of.

Press ⌘ CommandI for &quot;Get info&quot;
Unlock the &quot;Sharing and Permissions&quot; pane
Add your name by clicking the plus (+) symbol
Then select  &quot;Make username (Me)&quot; the owner


Apple has an excellent support document detailing these steps.
Using Terminal
It's actually much easier (IMO) to do this via Terminal   Just issue the following command (you'll need to enter your password at the prompt)
$ sudo chown -R &lt;YourUserName&gt; /path/to/file  

Where...

-R = recursively; it will take ownership of it and it's sub-folders (if any)
&lt;YourUserName&gt; is your login name.  If you're not sure issue the command whoami at the prompt
/path/to/file is the full path of the file or directory you want to take ownership of