If you have a Mac with MacOS X up to El Capitan (OSX 11), you can neither access APFS-formatted external drives, nor connect to a Mac in Target mode formatted as such. If your OS-harddrive is formatted as HFS+, you can only mount, read and write to other HFS+ disks.
Paragon wrote a driver in 2018, with which you might be able to READ a APFS disk, but it vanished not long after. I guess Apple wanted to entice people to upgrade to an OS later than El Capitan, or better, update to a newer machine, and therefore probably forced the driver out of open linkage.
This URL is fetched out of Wayback Machine, but apparently it is still on the company’s webserver: http://dl.paragon-software.com/free/Paragon-1218-FRU_MacInstallUnlock_1.0.164.dmg
Why did I write this? Because I despise being forced into a corner; because I take pride in keeping my old machines alive; and because I dislike their tactics. And to send a message. They might send ME one, I don’t know – it’s all speculation on my part – but then it’a all safely tucked away at Wayback machine. They can’t touch that, can they?