Save (Stored) MSFS FP are located in Hidden Folder:
C:\Users\yourname_\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState
but these are flt format.
To Load a regular FP in .pln format, just navigate to where you keep your Flight plans and load it in MSFS (using the save/load on the botto of the world map - or if you have a Fp .pln already created-navigate to your folder and select it and it will load in MSFS. You can also create a new flight (FLT format), in MSFS itself from the world map - select departure PAKT then arrival location PAJN, and it will load - then select FLY. Or you can build one in Plan G or Little Nav map, and export it as a FP for MSFS format, then load it from the save/load button on the world map in MSFS.
Actually, there is no new process at all in building or loading a FP in MSFS than it was in previous sims).