Converting a QGIS project to an OSM PBF file

I have a QGIS project with 3 layers:

1. OpenStreetMap layer
2. Polygon representing a specified area on the map
3. Lines representing streets in that area on the map

How do I convert this project to an OSM.pbf file?

I'm trying to then pass that file to osm4routing to process the street network of the specified area and output a graph ready for routing.