It’s an easy process. Make sure your Edge is synced with Garmin Connect. Find the course in Connect and click on the share option. Send it to another person, a text message link works well. The other person clicks on the link which pus it in to their own Connect. They then simply sync their Edge with Garmin Connect and that’s it. It takes a minute or so to do.
Of course you can transfer courses via GC, copying, emailing, etc., but it used to be you could transfer a course directly and wirelessly between Edge devices, no intervening computers, cables, internet, wifi, cell service, or anything else was needed. I've done this on a good number of occasions in the past, particularly at the start of group rides where I could transfer the course to other riders who didn't have it. Granted, it's been some years now since I last did this.