The common conception of satellite reentry is that they just burn up and are "gone". But the total mass must still be somewhere, right? What forms does it all end up as, and where?
The products of combustion, apart from gases, are small solid particles, 'ash', that slowly floats down to the lower atmosphere and, eventually, to the surface.
What is the destination for mass of a candle when you light it up? Same things will happen if some inbound spacecraft speed towards earth surface hitting air molecules. Only difference is the outer shell of spacecraft is too hard to burnout totally like a candle but remains mostly intact losing very few amount of mass by various forms including combustion.
During reentry on earth it collide with air molecules and heated up high enough to produce bright light visible. If same thing was not spacecraft but asteroid made by soft material it would burn out leaving gas and dust of various particles based on material it made on. Same thing would happen if any satellite fall from orbit and enter earth atmosphere.
Normally spacecraft do reentry process to return earth surface but satellite do not have such entry process in normal operation planing. Earth returning spacecraft designed with high temperature in mind and "gone" material during reentering process is relatively small which turned to gas if not fine dust. These materials ultimately falls on surface of earth if not light enough to float on air. But amount is really negligible to count.