How to make a Report Folder publicly accessible for all internal users in the org?

There's already a public group you can share the report folder with to do this.

  Following the help doc, you can go to Reports --> All Folders

Right click on the down arrow at the right side of the folder and select share Now, on the sharing screen, you can select Public Groups to share with. Search for All Internal Users and it should eventually display (might take a second). Select that group and set the Access to view. Now, all internal users have view access to that folder.