Normally your hosting environment offers a tool to backup your files and databases.
You need to have exactly the same version installed on both Joomla and Worpress. It's not necessary to have your license attached to both.
Th emigration from Joomla to Worpress (or vice versa) is a manual process. The only migration tool available is to migrate your data from v5 to v7 on the same server.
All documentation is accurate.