Share MacBook Ethernet connection with Nintendo Switch via USB
Is the Nintendo Switch already connected to the MacBook?  How?
Do you have more than one USB to Ethernet adapter?  If you don't then you will likely need more.
The Nintendo Switch is not a general purpose computer and so has very limited support for devices plugged in by USB.  It will support many USB to Ethernet adapters, so the easiest way to get a network between the two is to use USB to Ethernet adapters on both the MacBook and Nintendo Switch and connect them by an Ethernet cable.  An Ethernet crossover cable might be necessary but most Ethernet adapters can use the more common straight through cables in cases like this too.
This means your MacBook will have two USB to Ethernet adapters, one to where you get your internet, and the other connected to the Nintendo.  You will have to set internet sharing to share from one to the other.
It might be cheaper and easier to use some kind of Ethernet switch, hub, or access point instead.  Without knowing the situation you are in this might not be possible.