Maps for walking countryside routes

Having a look at creating and using offline maps for walking in the wilds - on to & along Pennine Way in Cheviots (UK).

Discovered that I cannot always use paths that are shown on o.s. maps and cannot input latitude & longitude info for destination, when path not recognised as a path on Apple Maps. (path/track can be seen on satellite map, but cannot appear to use for route)

Is this possible to do or I am limited by what paths Maps will use?

Also cannot appear to ‘add’ to route, after destination, to increase walk to make a circular route.

Is this possible?

If what I’m attempting is not clear, let me know!