My layout was built in 14 separate modules. After construction it was disassembled, shipped and reassembled in my train room. I am including a couple of pictures taken at delivery to show how this was done. I have many more pictures if interested but I feel these show the general idea.
The first picture shows three different situations. Lower right is where a turnout spans the module joint. Top right is a complex vertical and horizontal easement location so a large section of preformed track was removed. The other joints are level so the joint was at the module joint, the rail joiners were slipped on, soldered and the the ties slipped under the track joint.
The second picture is part of the yard. All the track joints are at the module boundaries. The track inside the roundhouse is part of the RH base.
Remember no wires can cross a module joint. There should be a terminal strip on each module with jumper wires under the module joint connecting the terminal strips.