Splitting the cable to a Y would work if it was a CAN communication bus which is multidrop (multiple nodes on the same wire). But ethernet, along with many other protocols, is point-to-point, meaning you can’t simply split the wires to connect one device to two…
There is a way of kinda doing that, by using a switch and connecting your camera to one port, then your raspberry pi to another port. Then you could use a port mirroring feature to send a copy of all the data received from the camera into a third port, to which you could connect your herelink.
You could do this but you’d still have to deal with the fact that your camera is sending to the IP address of your raspberry Pi, and so the mirrored traffic sent to the herelink may just be dropped by the herelink (I’m actually not sure what would happen in that case).
The solutions such as RTSP and multicast were designed to allow this kind of thing, streaming from one device to multiple, but they require a bit of knowledge to setup. In other words, I can’t really give you a simple answer here, you’re going to have to study RTSP and speak with your camera manufacturer about how to setup RTSP.
Then, you’re going to need to find a switch to connect all the devices to. We have run a test with SwitchBlox Rugged using RTSP and it works. Take a look at the last post on the thread below.
Apologies, this is not a simple answer, but this isn’t a beginner level thing.