I would like to mount an Apple iPad to my Linux device, to make a jpeg or ddrescue recovery on it. How I would do this with an Apple device?
You can't access the block device on Apple directly, it is forbidden by the OS, on which you don't have a root access, despite that you've purchased it and it is yours. To be able to do these, you have to jailbreak it (I intentionally don't use the word "crack", because it is your property). It is hard. Although the OS of the Apple mobile devices, the iOS, is based on a Unix variant (OS X, which is based on freebsd), it doesn't mean that you would have the freedom of the unixes on it, its exact opposite is the truth.
Better solution would be to use some application-level thingy (i.e. to copy the files with usb or wifi).
If you have some electronical affinity, also soldering out the ssd chip from it would be a solution, although it doesn't solve the problem that their whole disk is encrypted (note: being the legal owner of the device, you are still not allowed to decrypt its content).