If the routes are saved in a named folder, all the routes in the folder can be displayed on the map. When viewing the map, folders are listed in the panel to the left (west!); click on the folder name, the component routes in the folder will be listed underneath; at the bottom of the list click on "Show all on map".

The routes will have separate identifying colours, and the details of each route are highlighted as you scroll over them. Clicking on a route of the map opens it on its own for editing etc.

Note I think there is a limit (50?) in the number of routes per folder.

Supplementary: I appreciate the method above is not the same as actually joining routes together. See previous thread on this - "Joining two routes?" dated 15th Aug 2023; it is on Richard's longlist, but not easy to do.