Each router in your network can be used as a default gateway by your hosts but the problem is that most hosts only support a default gateway.
That’s why we use “gateway redundancy” protocols like HSRP, VRRP, and GLBP. You can read more about this here:
About the switch…A switch is a layer two device so all it “cares” about is forwarding Ethernet frames. It only cares about looking at the source MAC addresses to learn addresses and looking at the destination MAC address to figure out where to send it to.
ARP is just one protocol that you can find in an Ethernet frame…an Ethernet frame can also contain an IPv4 packet, an IPv6 packet or some other protocols.
ARP is used to bind a layer two address (MAC address) to a layer three address (IP address). We only need this on a switch if you access the switch with telnet/SSH or if you configure your switch as a router (that’s a layer three switch).