The main frame is of 3” wide Narra. The corners are mitered lap joints for strength as the mirror glass itself weighs over 40 lbs.
The ‘outer’ frame is notched to accept a 1” wide by 1/2” Kamagong (Ebony) inner frame for decorative accent.
The Kamagong inner frame has lap joints and fits together this way.
And here’s the sketch-up drawing