As a Linux admin, I am thoroughly confused. Are you saying that MacOS maintains a separate routing table for each interface?
Typically, a system has only one default route. You can have many interfaces and many routes, but only one default. Otherwise you don't know which default gateway to send a packet to.
If you bind to a specific interface it will use that default gateway:
fenrir:~ $ traceroute -ni en0 google.com
traceroute to google.com (216.58.194.142), 64 hops max, 52 byte packets
1 172.16.22.254 0.565 ms 0.340 ms 0.300 ms
2 172.16.25.60 0.893 ms 0.727 ms 0.721 ms
3 172.16.25.0 0.954 ms 0.819 ms 0.815 ms
4 108.218.244.1 7.422 ms 1.908 ms 3.617 ms
5 *^C
fenrir:~ $ traceroute -ni en2 google.com
traceroute to google.com (216.58.194.142), 64 hops max, 52 byte packets
1 192.168.88.1 12.399 ms 3.768 ms 0.811 ms
2 192.168.80.1 1.950 ms 8.322 ms 1.702 ms
3 172.26.96.161 30.448 ms 393.449 ms 46.537 ms
4 107.72.199.60 182.826 ms
107.72.199.36 34.835 ms
107.72.199.60 50.557 ms
5 12.83.186.101 52.178 ms 32.536 ms 33.431 ms
6 12.83.186.85 38.516 ms 51.138 ms 61.687 ms
7 12.122.5.190 48.626 ms 251.733 ms 38.487 ms
8 12.122.2.197 58.184 ms 82.183 ms^C
You can also also do this do this by IP address
fenrir:~ $ traceroute -ns 192.168.88.243 google.com
traceroute to google.com (216.58.194.142) from 192.168.88.243, 64 hops max, 52 byte packets
1 192.168.88.1 4.658 ms 10.498 ms 3.633 ms
2 192.168.80.1 4.264 ms 58.660 ms 7.153 ms
3 172.26.96.161 105.557 ms 41.207 ms 32.243 ms
4 107.72.199.60 66.562 ms
Which interface is used when IP address / interface is not specified is selected by the Service Order setting in the Network control panel.
Typically, a system has only one default route. You can have many interfaces and many routes, but only one default. Otherwise you don't know which default gateway to send a packet to.