FL12, like other Framework Laptops, requires a USB Power Delivery charger. Although 5V3A is a PD rating, most if not all chargers have at least a higher rating like 9V2A the laptop will choose that and most 5V chargers don’t support PD protocol. It’s possible to use 5V3A PD under very specific condition like this example.
FL12 is likely using the same NVDC topology like FL13 shown here, the bypass is not enabled. The battery and the system rail are connected in parallel, downstream of the buck-boost, so there’s no “prefer”. It’s more like two water tubes one input the other output with the battery as the water tank in between.
Note: this is supplementary information not nitpicking your comment
If you have a PD charger for your smartphone it could be enough for the FL12. If you are looking for a newer one, 35W and over will charge the laptop while gaming, 60W and above will charge the battery at or near full speed. Make sure to read the voltage and current as well as wattage as some smartphone chargers use both proprietary protocol for its smartphone and PD protocol for other devices, the power for the latter is likely smaller.
I don’t have an FL12 but I have an FL13, according to my testing it supports 9V2A, 9V2.22A, 12V1.67A, 12V3A, 12V5A(very rare charger rating but works) 15V3A, 20V3A, 20V3.25A, 20V5A. The less powerful FL12 can negotiate 100W but draws at a lower wattage, which is normal. Using a low wattage charger will discharge the battery at high workload but won’t cause any malfunction.