The pipes must of course be properly sealed, not with the plastic plugs sometimes supplied. O-rings are more for the consumer sector for detachable quick connectors.
How do you install refrigeration lines retroactively in empty conduits? I imagine it gets tricky at certain bends at the latest. We're talking about the usual copper pipes, right?!
Well, basically not at all, at least not when it comes to bends. It already becomes difficult in the smooth-walled installation pipe, not to mention the flexible one.