“GMail takeout files” aren’t a thing. mbox is a standard file format from Unix. Some Unix-likes provide the mail command and on others you have to install s-nail, after which you can do mail -f {path to mbox} to view all mail inside. If there are multiple files, you can literally just concatenate them like so: cat /somedirectory/*.mbox > theultimate.mbox.

Apart from that, most GUI mail clients allow viewing mbox. When the mbox file updates, the client says you have new mail. For example, in Claws Mail, skip setup, then go to plugins and enable the mbox plugin (while you are there, may as well enable pgp, s/mime, calendar, dillo to handle all formats). Then, go to create new account, and in the “Basics” tab, select “local mbox file” for “Receive”. It will then ask for a file path.