I can’t specifically speak to the Framework, but generally with laptops and USB-C charging you can still use them with lower wattage chargers. If you’re not using the machine at all it should still charge, just slower than it’s capable of. If you’re using the machine but not intensively it might still charge, but slowly. If you’re using more power than the charger will supply they’ll usually supplement from the battery. Depending on the workload it can be a momentary surge or continuous. If you’re continuously drawing power from the battery and charger you’ll run the computer longer than you would on battery alone but you’ll still eventually need to switch to an adequate charger or stop using the computer while it charges.
I would be shocked if 65w doesn’t work at all. How much you can do with it will be variable. It should be adequate for charging at the very least.