What components are inside a port expander, how does it work? (Are they tiny MOSFETs?)
, do they use MOSFETS internally to switch the Pins high/low?

probably? Most digital circuitry these days is CMOS.

I have noticed that the Port Expander is not a perfect short circuit to GND and have measured a voltage drop between the pin of the Port expander and GND, this voltage drop increasing when a higher current sunk through it to GND

Well, that will be specified in your port expander's datasheet. Reading that gives you more information than speculating what kind of transistors are used on the die of that IC.

In the MCP 23017 datasheet, (under param No. D080) it says when MCP is operating at &quot;Output-Low-Voltage&quot; the voltage drop across the MCP23017 pin to GND will be 0,6V when the Iol (sunk current) is 8mA and the Vdd (supply voltage of MCP) is 4,5V. This would explain my Problem.

Exactly!

Im looking for literature / links explaining the basics of (I/O) Port Expanders which i can reference in my bachelor thesis which explain this.

There's nothing you need to explain in your bachelor thesis: the low-pulling output transistor, whatever kind of transistor that is, has a non-zero forward voltage under current. Like anything in this universe that isn't a superconductor (I'm sure your advisor is familiar with Ohm's law...).

But from my understanding a MOSFET has a set (Rds on) resistance when on, but the Port expander Resistance changes based on the current sunk or sourced, why? So do they internally use Mosfets?

Again, whether or not they internally use MOSFETs or not doesn't matter to your application: every transistor has an effective resistance that is not zero. The question of how close you get the resistance to zero is a question of size, speed and cost. The result of the design is what is specified in the datasheet - nothing more to add to that.