Microcontroller maximum and minimum source and sink current
We’ve been over this. The GPIO source and sink capability depends on the ‘on’ resistance of the P and N driver FETs, respectively, a topic I and others have already addressed in your previous questions. That parameter, Rds(on), isn’t in the datasheet, but can be derived from V(oh) and V(ol) at a given source or sink current.
Another name for this parameter is called drive strength.
Drive strength depends on two things:

Process characteristics
FET physical size

Drive strength is ultimately a statement of guaranteed Rds(on) that will satisfy the stated V(oh) and V(ol) at a specified source or sink current.
Why do they state it that way, as V(oh) and V(ol) rather than Rds(on)?
When the IC is manufactured, they test drive strength by putting a known source or sink current load on the pin and measuring the resulting V(oh) and V(ol), only accepting parts that are ‘strong’ enough to meet the datasheet spec, plus some margin. In other words, they use a functional spec that meets the system requirements to make valid high or low outputs.
Nevertheless, think of it this way, like a chip designer would: a FET is a resistor, which is in one of two states: infinite resistance when off, or at Rds(on) when on. It’s not more complicated than that. The fact that FETs behave like switchable resistors is one of their benefits over bipolar: it allows connecting them in parallel to reduce the overall Rds(on).
And some microcontrollers do exactly that, and further, allow selectively enabling parallel FETs to alter drive strength. This is helpful to reduce power dissipation and switching noise, or to match drive impedance to the system.