hydroxychlroquine faciliates Zinc entering cells. once zinc is in the cell, it slows down virus replication.
azithromycin has some synergies with HCQ, but it also fights off other infections that could hurt you while your body is busy fighting the virus. the time that the zinc buys you allows your white blood cells to kill the virus before it gets too big in number.
This seems to be the simplified reason as to why it might possibly be working.