Making multiple files or a folder available offline in Google Drive
Select all files (automatically using the menu option), then slide the "make available offline" switch. It will keep all the selection on device.

You can select a folder but Google will refuse to "make available offline".  Instead go into the folder and select any number of files for syncing, and the parent folder will of course also now be created.

Finally there is an option to automatically make recent files offline, but I don't know whether those files are then permanently made available, or only for a couple of weeks for example.