Well, you either transfer it directly from A to B using a protocol that both ends support such as Kermit (what we used to use back in the day for copying files

Well, you either transfer it directly from A to B using a protocol that both ends support such as Kermit (what we used to use back in the day for copying files from bulletin boards using modems), or FTP, or SMTP (yeah, e-mail attachments), or via some intermediatory server or device such as Google Drive or a USB stick (floppy disk). I guess nowadays the only files that most non-techies care about are photos and videos on their phones, so solutions specific to those work, or indeed the ubiquitous e-mail attachments.