Plasma contains molecules that contribute to immunity. The donation transfers those molecules from the donor to the recipient. It doesn't directly confer any immunity to the recipient, it reduces the amount of virus circulating in their body, giving their own immune system more time to respond.
I am in an open source Christmas lights community that sources pixels from China. About a month and a half ago one of the Chinese vendors posted that the PRC is full steam ahead on Plasma therapy. He also stated his elderly father had gotten covid and was recovered within 24 hours.
Source: https://nos.nl/artikel/2330036-nederlandse-coronapatient-hee...
NB: I am NOT a microbiologist and do not know how far this is related.