Use leaflet to create personalized routes

I am new to OpenStreetMap and I would like to use it in a web application, where I would let the user choose preferred options (like travel mode) and even use tags to create a personal route. I've seen leaflet could be a good choice for an interactive map, but I couldn't find any guide or tutorial that shows how to "query" OSM to plan routes using tags: let's say, a user would like to go from marker A to marker B travelling on a route where there's a node "shop=bakery"; it is possible to do that? How?

Also, I have some other doubts:

• I've tried working with open trip planner (using the jar and running it on localhost) and I've downloaded GTFS for Venice, but what do I have to do if I wanted to work with multiple locations?
• Since I have to download also OpenStreetMap data for the same region as the GTFS file, again, how it is possible to integrate all the files to, let's say, visualize the roads and create travels on an entire nation?

I know this is a lot, but I really don't know where to start. Any help, tutorial,guide link or example project would be truly appreciated.