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No, you can use the same one for all VMs (assuming you want all to connect to internet). Windows host will also connect using this virtual switch.
I can't read you language but presumably the first picture says "Enabled, Bridged", the second is the name of your network and the third is "Network Cable unplugged". You should have a network bridge with an icon like this - perhaps it is cut off in your picture.
See this for more details Hyper-V Virtual Switch Manager
Exactly. In addition, it would not even be possible: One host NIC can only be used for one external switch meaning that you can have at most as many external switches as you have network adapters on host. Normally this means that you can have two external switches, one for the Ethernet NIC and another for the Wireless NIC.