Route Creation
I create routes using a freeware Windows application called ITNConverter (ITNConv). Completed route files are exported from Windows/ITNConv and imported into and old android phone for the android GPS application, CoPilot GPS, for realtime GPS based guidance while enroute.
ITNConv can be downloaded from many internet sites. Here's one download site I've used: https://www.filecroco.com/download-itn-converter/download/
ITNConv requires Microsoft Internet Explorer to be installed on your Windows system. Some newer versions of Windows apparently don't have it installed. I've not encountered this problem but this requirement has been noted by others. You may need to install it.
Windows occasionally gives a security warning about running ITNConv. Ignore this warning, ITNConv is completely safe.
ITNConv.exe's file size very small (~2MB). Therefore, it can be replicated throughout the file system without consuming much disk space.
Multiple instances of ITNConv will run simultaneously and independently. This is handy when creating routes so you can refer back and forth from one route to another.
ITNConv.exe creates and modifies a unique settings file (ITNConv.stg) in the directory in which ITNConv.exe is executed. This important file contains all ITNConv preferences as well as the "favorites" you create.
ITNConv created routes can include an unlimited number of "waypoints" through which the route will travel and Copilot GPS will accept routes created by ITNConv with an unlimited number of "waypoints". My routes often have fifty or more waypoints which guide the route to exactly the path I desire.
(I have an optimized file structure based on the above that is great for my use but probably unnecessary for the casual user :-).
Real time Guidance
While en-route, I use an old android cell phone for a GPS. Google chose to restrict app to app file system access in Android 11 (API level 30). You can still move files around in later versions but it requires a few more steps. When selecting your old phone for a GPS, try to pick one with a large display that uses Android 10 or earlier (but not much earlier). I have millennial children who've cast off many still serviceable old android phones from which I can choose.
For realtime guidance software, I use an android based GPS application, CoPilot GPS, which can be installed from Google Play. Over a decade ago, I purchased a perpetual license for Copilot consequently, I'm unfamiliar with the ownership options available today. I believe there's a free version available.
https://play.google.com/store/apps/details?id=com.alk.copilot.mapviewer .
There is also an iOS version of Copilot that I have personally never used. I don't use Apple products.
https://apps.apple.com/us/app/copilot-gps-navigation/id504677517
Moving Route Files Between ITNConv and CoPilot
Once a route file is created using ITNConv it must be copied onto the android phone. There are lots of ways to move files back and forth between Windows and android. It can be as simple as emailing the route file to yourself.
I choose to use a freeware android file manager, TotalCommander (TC), which can be installed from the Google Play Store. When installing TC, also install two TC plug-ins, TC LAN plugin and TC FTP plugin. TC is quite extensive, with a myriad of capabilities. Install it, fool around and ask questions.
Considerations
When creating a route in ITNConv, first add your route's beginning and end points to the itinerary and "get directions". To modify the route, hover the "hand" over the route until it becomes a pointing index finger and click. This will place a "Waypoint" on the route. Then, drag the Waypoint to wherever you'd like the route to go.
ITNConv and Copilot have differing routing algorithms and hence may choose different roads to get from one "waypoint" to another. It is necessary to use a lot of waypoints to force the routing algorithms to calculate the same route. Once in Copilot, review your route in detail to ensure that it is what you intend.
When exporting routes from ITNConv intended to be used in Copilot, always choose the export file format: "CoPilot 10 (.trp)". Other versions are mildly incompatible.
CoPilot has two possible route_through locations: "Stop" or "Waypoint". Waypoints, pass right through, with Stops, CoPilot expects you to stop and interact with your GPS, then proceed. Waypoints are much more commonly used.