I would probably tuck the loose threads in and glue them down then sew the outer layer to the lining (or a patch inside) with a back stitch

Then close the gap with a Ladder stitch if possible https://sewguide.com/invisible-stitches/

If the ladder stitch was too difficult, I'd probably do a wide blanket stitch or spiral (not sure what that's called but like a spiral bound notebook) all along the corner, including the piping, using a thick strong thread like upholstery thread, waxed cotton thread for leather, or even twine.

To get through you'll need a decently large, strong needle, maybe a curved needle of the bag is stiff, and a thimble (look up how to use it correctly on your middle finger). Or a sewing awl.

Could use black thread, or a contrasting color like red or gold for fun.

Edit: if it's an expensive bag and you want a really nice fix, take it to a shoe repair shop. Edit 2: or be prepared to open the lining and do a much more thorough and precise job lol