Hello Po
You have described several different and separate mechanisms that are used to deliver redundancy. First, you mentioned Active/Standby firewalls, which are a setup that has its own redundancy mechanism. If you use this, then you don’t need to use HSRP or VRRP since Active/Standby has its own redundancy operation. You can find out more about this feature at the following lesson:
The above Active/Standby feature is similar to first hop redundancy protocols (FHRP) in that the active device adopts the (virtual) IP address that is used as the default gateway. If that device fails, the standby device becomes active and adopts the IP address. That’s how all FHRPs operate. This means that you don’t need to configure anything else at the switch that connects the two routing devices! That’s the magic of these redundancy protocols.
How does that work? Well, let’s use your diagram for reference. The links to Router0 and router 1 are connected to Layer 2 interfaces on the switch. As soon as one router takes over from the other, it adopts the virtual IP address. The newly active device will send out a gratuitous ARP which is essentially a message letting all devices on the network know that the virtual IP address now corresponds to its own MAC address. Thus, all the hosts on the network will now be sending their traffic to the new MAC address, so the switch knows on which port to send such frames.
EtherChannel should only be used when you want to share traffic across multiple links. In your scenario here, this is not the case, so it would not be beneficial.
I hope this has been helpful!
Laz