The first part is correct, mostly it isn’t “much much” higher because it is wasted performance but you could hace a 24port switch with the CPU of a 48 port switch and you could have over 50GBit internal switching bandwidth for the 24 ports.

The Second part is a bit strange for me. Probably because of the wording. When you say modem it is probably already a router because you have multiple LAN ports. A Modem normaly only supports 1 WAN and 1 LAN Port at consumer level devices.

You can have routers behind routers but unless manually configured correctly on the main router and the 2 routers WAN the 2 LAN Networks behind each router can’t reach each other, like you cant easily reach your neighbors PC unless he opens a connection to it specifically.

Whats the model number of the thing you called a Modem?

Can you extend on what you meant when you statet “which I assume would be minimum”

And without and even in some cases with expensive load balancers you cant say stuff like “use bandwidth that is left by IoT” for example. It is very random who gets more or less bandwidth of a connection when it is at full capacity. Because of the way TCP was designed in the beginning. Resiliency was much more important than fairness ;)

No switch or router does load balancing, you wont get 5 times 20mbit it will be all over the place…

Ok I guess we have to start way in front of your question to not just answer your question but give you an understanding.

First of all, what does a switch do: A switch has for example 24 ports and all ports are connected to the CPU of the switch. When a pc connected to the switch wants to reach the router to access the internet he asks the switch to really the data to the router but the switch doesn’t know where the router is so it sends on all ports „Hey are you 192.168.0.1(for example if it’s the gateway in your network config)“ then on one port the router answered „yes“ and the switch makes a note on a list that this up is on port x. So over time the switch knows who is where and what speed does the port support. Because said CPU has a bandwidth normally greater than the sum of all ports (rock a Unifi 24 port switch by chance and it had 26gbit bandwidth) it can for example connect 2 pcs transfering files with a full gigabit all while running a download from the internet router to a third pc with no problem. So as long as you don’t want to access one ressource with multiple PCs you have full speed and if you do the packets are stored in a small buffer and if it is full the PCs are told to wait a millisecond and it is transfered in a first come first serve method so everyone can get data through.

All a router does extra on top is have an extra list if he doesn’t find the receiver of the packets internally he notes the address of the PC that asked and sends the request to the wan port and when someone responds he know who to send the response internally.