How does PING command find the required network interface?

I'm using the ping command pretty often, but now when learning about network interfaces come to misunderstanding of how the desired interface is determined when pinging the required IP. I have the following network interfaces:

$ ifconfig
lo: [...]
      inet 127.0.0.1  netmask 255.0.0.0
      [...]

eth0: [...]
      inet 20.20.20.20  netmask 255.255.255.255
      [...]

virbr0: [...]
        inet 192.168.122.1 netmask 255.255.255.0


 //lots of other interfaces

Example 1.

I decided to run strace ping 8.8.8.8 to understand what we actually have under the hood and what I've noticed the following fragment:

socket(AF_INET, SOCK_DGRAM, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(1025), sin_addr=inet_addr("8.8.8.8")}, 16) = 0
getsockname(5, {sa_family=AF_INET, sin_port=htons(46754), sin_addr=inet_addr("20.20.20.20")}, [16]) = 0
close(5)

The question is how ping understands that 8.8.8.8 can be reached via eth0 which my device has ip address 20.20.20.20?

Example 2.

When pinging my QEMUVM sudo strace -f ping 192.168.122.77 I got

socket(AF_INET, SOCK_DGRAM, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(1025), sin_addr=inet_addr("192.168.122.77")}, 16) = 0
getsockname(5, {sa_family=AF_INET, sin_port=htons(33866), sin_addr=inet_addr("192.168.122.1")}, [16]) = 0
close(5)  

and it was also somehow undertood that the interface virbr0 should be used.

QUESTION:
How is it determined which network interface to use when ping-ing a specific ip address? Maybe I missed some dedicated syscall?

This is handled by the routing layer in the OS. While it can be trickier in practice, the basic idea is that the target address is tested against the routing table, and the most specific route that matches is used.

For example, anything to 127.0.0.0/8 is routed, explicitly, to "localhost" or often the lo-type address.

The most general route is 0.0.0.0/0 which is "everything" and has the lowest precedence. Packets going here go through the defined "gateway", otherwise known as your upstream router.

Somewhere in the middle is your local network, like 192.168.0.0/24 or something like that, or in some cases multiple local networks depending on your config. Packets going here are sent out the matching interface directly to the destination machine, no gateway is required.

Note that if you're pinging certain networks and want to use a certain interface, like your wired instead of wireless, you can bind your socket to a specific address before using it.

Since your computer usually has at least two interfaces, localhost and some kind of other network connection, ping programs do need to select the correct interface or the packets won't be able to be routed. For example, pinging 127.0.0.1 out your wireless connection will go nowhere, while pinging a network address on the loopback interface is likewise pointless, no device will respond.

If you're writing your own ping, you may want to carve out a special case for localhost pings and bind accordingly. A more robust implementation checks the routing table, and picks the correct address to send from. Many tools also have a -i type interface specifier to specify either the source IP or the source interface.