RethinkDB doesn't do any planar projections of spherical coordinates (if that's what you are asking for). There are many independent tools available to perform such projections if you need to draw a map or convert to a Euclidean coordinate system. As coffemug noted, WGS84 is not a projection but just a reference ellipsoid used for distance calculations along earth's surface. (Edit: Ok, I guess that can be considered a projection as in projection from earth's actual geometry onto an ellispoid. It's not a projection with respect to latitude and longitude though, if that makes sense.)
> Along those lines, are distance calculations done on a geodesic
Yes, along the geodesic between the two points on an oblate ellipsoid. We use the algorithm from Karney 2013, see http://charles.karney.info/biblio/papers/karney13-geod.pdf
> geometry.distance(geometry[, {geoSystem: 'WGS84', unit: 'm'}]) Does this do a correct interpolation of degrees to meters along the path (again based on the projection)
Yes, again using Karney's algorithm (but not by doing a projection first).