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The brown dashed are footpaths. you would be able to engage, but in the case of a breton coastal singletrack i wouldn't recommend it.
The white dashed lines will be farm/exploitation tracks. Most likely you would be fine though it could be fenced with no entry signs as it doesnt come with a right of way. Also they don't "connect" as well as in the Uk so you might find that it doesnt help progression on a long distance ride.
The vibe depends on the place in france. Somewhere remote would be fine, vineyards areas would depend on season, etc.to give some substance in case you're interested, in france the medieval structure has disappeared due to famous events, but also all farmland was rationnalised and redistributed after the war. the was a big game of swaparoo wherever possible. So there isn't an equivalent to old roman roads and cycle routes are down to local and greater (eurovelos) initiatives.
this map gives you the pedestrian GRs, which could be cycled.
https://www.google.com/maps/d/viewer?mid=1nh_jR073YQFIYoMudLxSZXoTLUo&ll=46.75920466873938%2C2.2992418074709993&z=7
Things are different in france. Any bike path and quiet route will be on rwgps so it is your best bet. In my recebt experience, anything that will be bike-ready will be shown on the osm cyclemap layer.
However in france there are no bridleways and connectable right of ways to speak of, as landownership is different. You can venture on footpaths, forest and farm tracks, but potentially can be booted out as that will be private.
GR (grande randonnée) pathways which are long distance pedestrian hiking trails are not designed for bikes.