What CRS is my data being transformed to in ST_Project? What about ::geometry?

My understanding is that ::geography transforms data from whatever CRS into 4326. But what is ::geometry doing?

Relatedly, in the context of the ST_Project function:

st_project(a.geom::geography,b.recommended_radius,radians(a.azimuth))

which takes as input a Point (in 4326), a distance (in meters), and an azimuth (in radians), presumably there’s a transformation of some kind happening to get from a Point(4326) to a distance (in meters) – is there a way of finding out more details about the function? The documentation is minimal.

Ultimately, the ST_Project function is nested inside of an ST_MakeLine function, then cast back to ::geometry (but without any specified CRS).

st_makeline(array[a.geom,st_project(a.geom::geography,b.recommended_radius,radians(a.azimuth))::geometry])

Finally, for context, I am drawing wedges around a given azimuth using this code:

st_intersection(
    st_buffer(
        a.geom::geography, b.recommended_radius*1000, 50
    )::geometry()
    , st_makepolygon(
        st_makeline(
            array[
                a.geom
                , st_project(a.geom::geography, b.recommended_radius*1000*2, radians(a.azimuth-a.sector_width/2))::geometry
                , st_project(a.geom::geography, b.recommended_radius*1000*2, radians(a.azimuth+a.sector_width/2))::geometry
                , a.geom
            ]
        )
    )
)