Regarding road classification based on the type of populated place that it links, often times it will be trunk for cities, primary for towns, secondary for villages, and tertiary for hamlets. However, it is difficult to completely base a highway classification scheme around that since occasionally you will have towns that serves as major highway junctions or villages with little to nothing going on in them to warrant a secondary route.

Looking at the pages for primary, secondary, and tertiary classifications on the OSM Wiki:

The US primary definition is extremely vague, stating simply "Primary highway or arterial road." "Primary highway" could mean anything, from a primary state route, the primary road through a region, a primary arterial, etc., and so could "arterial road", as an arterial is simply a major road through an area.

The page for secondary is almost wholly Euro-centric, and lacks any table for country-specific definitions.

The US tertiary definition seems to be the only coherent one out of these, implying it could be for hamlet-connectors or urban/suburban collector roads.

These pages (or at least the ones for primary and secondary) definitely need to be reworked, but each of them are their own separate projects that need to be heavily discussed with other highway-oriented mappers throughout the US to get a consistent definition for how it should be applied in the country, and shouldn't be changed as the result of this discussion or at the discretion of a single user.

As road number-based classification, road should not be classified based on what its number is, since most of the time, especially for state routes, the number just implies how the road is funded. It definitely can be an indication that a road is a major highway, but it is very common to have secondary state routes and county roads play a role similar to what a primary highway would or for primary highways to be dead-end roads just shy from being literal driveways.

All that aside, I think that these roads should neither be classified based on vague/obsolete Wiki definitions nor their road number. I think having two dead-end secondary routes is confusing but primary might be overkill, at least for this alignment. It might be better to have either a secondary or primary route pass through here, but along Esworthy Road, rather than taking an unnecessary deviation towards a golf course. I doubt most traffic driving between Poolesville and Potomac will take a detour just to stay along state routes, and it looks like a lot of traffic does use Esworthy as a shortcut judging by tire track patterns visible from the aerial imagery and in street-level view. However, through trucks are prohibited on Esworthy Road, so I'm not completely sure.