OSM needs no data plan, just the GPS signal and updated maps downloaded from either Organic Maps (the easy way); or OSMand, if you plan to do serious tasks on mapping your town for instace.

to be fair, doesn't google maps offer the same? Have downloaded offline maps of cities while traveling without a data plan

Dowlading maps it's far easier under Organic/OSMand. You choose the country and/or the region, and download them. That's it.

look up streetcomplete and everydoor. They are much more targetted towards helping improve OSM than OSMAND.