1991 Citroen XM V6 break renovation

[Dieselman]

Good news on hearing it is running, not so good o the floor area, but most XM don't rot the floor panels, so obtain a cut from another scrap car and weld the complete panel in.
Either the floor was damaged underneath and left, or there has been water entering the cabin for a long time and sitting under the soundproofing.
Did you scrape out the seam sealer, or was it already missing. That could be where water has been entering.

Jacking the car up under the floor damages that seam and unless properly repaired will lead to rot.