How to exchange files between Mac Studio M1 Max and iMac 10.1 when AirDrop and DropBox fail?

How do I exchange files quick and easy and reliably between a Mac Studio M1 Max, Sonoma 14.8.3 and an iMac 10.1, El Capitan 10.11.6 when AirDrop does not work and file-sharing with DropBox is only hit and miss? Both computers are next to each other. Thank you for your time and patience.

[Re-Titled by Moderator]