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How to forward SSH access of one machine, through another, to the rest of a network?

I have a setup with two Raspberry PIs, a CM3 module with only a USB connection to the outside, and the Pi4 with a lot of connectors.
~~I set up things such that CM3 functions as USB Gadget, and once connected to one of the USB jacks on the Pi4, I successfully logged into the CM3 with SSH - from a console on the Pi4.~~
Now, the Pi4 is connected, through its LAN jack, to a small network.
I would like to be able to access the CM3 from devices in that network (e.g. log in with SSH).
So I guess, I need to set up some sort of forwarding within the Pi4 in order to do that. I'm not sure of the correct terminology for that - looked for "bridge" tutorials, but what they did seemed not what I need, or perhaps I only found unfitting tutorials.
What do I need to do to set this up?

I have a setup with two Raspberry PIs, a CM3 module with only a USB connection to the outside, and the Pi4 with a lot of connectors.
I set up things such that CM3 functions as USB Gadget, and once connected to one of the USB jacks on the Pi4, I successfully logged into the CM3 with SSH—from a console on the Pi4.
Now, the Pi4 is connected, through its LAN jack, to a small network.
I would like to be able to access the CM3 from devices in that network (e.g. log in with SSH).
So I guess I need to set up some sort of forwarding within the Pi4 in order to do that. I'm not sure of the correct terminology for that—looked for "bridge" tutorials, but what they did seemed not what I need, or perhaps I only found unfitting tutorials.
What do I need to do to set this up?

raspbian networking

raspbian networking ssh