Visualization via Portainer:
The docker0 bridge is virtual interface created by docker, it randomly chooses an address and subnet from the private range defined by RFC 1918 that are not in use on the host machine, and assigns it to docker0. All the docker containers will be connected to the docker0 bridge by default, the docker containers connnected to the docker0 bridge could use the iptables NAT rules created by docker to communicate with the outside world.
$ ifconfig
br-31710a035769: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.21.0.1 netmask 255.255.0.0 broadcast 172.21.255.255
inet6 fe80::42:1ff:fea1:967b prefixlen 64 scopeid 0x20<link>
ether 02:42:01:a1:96:7b txqueuelen 0 (Ethernet)
RX packets 14 bytes 2023 (1.9 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 7 bytes 826 (826.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
br-7722e6280c94: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.19.0.1 netmask 255.255.0.0 broadcast 172.19.255.255
ether 02:42:a5:75:db:16 txqueuelen 0 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
br-d65fd7576eda: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.20.0.1 netmask 255.255.0.0 broadcast 172.20.255.255
ether 02:42:ef:44:d1:8b txqueuelen 0 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
docker0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255
inet6 fe80::42:57ff:fe98:606 prefixlen 64 scopeid 0x20<link>
ether 02:42:57:98:06:06 txqueuelen 0 (Ethernet)
RX packets 1991 bytes 718667 (701.8 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 1926 bytes 519919 (507.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
docker_gwbridge: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.18.0.1 netmask 255.255.0.0 broadcast 172.18.255.255
inet6 fe80::42:50ff:feba:99d3 prefixlen 64 scopeid 0x20<link>
ether 02:42:50:ba:99:d3 txqueuelen 0 (Ethernet)
RX packets 512 bytes 356446 (348.0 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 653 bytes 147926 (144.4 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.0.23 netmask 255.255.255.0 broadcast 192.168.0.255
inet6 fe80::ece8:d1a0:af88:499 prefixlen 64 scopeid 0x20<link>
ether dc:a6:32:0a:ed:2a txqueuelen 1000 (Ethernet)
RX packets 1304281 bytes 1518840155 (1.4 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 754363 bytes 249836692 (238.2 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 46941 bytes 4301574 (4.1 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 46941 bytes 4301574 (4.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
vethd192713: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 169.254.76.63 netmask 255.255.0.0 broadcast 169.254.255.255
inet6 fe80::f46c:895:f5c2:b93b prefixlen 64 scopeid 0x20<link>
ether 62:96:0c:27:a2:09 txqueuelen 0 (Ethernet)
RX packets 74 bytes 6988 (6.8 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 445 bytes 149066 (145.5 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
vethd56ba56: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 169.254.26.89 netmask 255.255.0.0 broadcast 169.254.255.255
inet6 fe80::69e0:1379:686f:ff87 prefixlen 64 scopeid 0x20<link>
ether 7e:0b:9f:1d:90:d8 txqueuelen 0 (Ethernet)
RX packets 1987 bytes 746125 (728.6 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 2414 bytes 695808 (679.5 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
vethfc7c41f: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 169.254.186.128 netmask 255.255.0.0 broadcast 169.254.255.255
inet6 fe80::8d9a:c2ca:d220:f7a7 prefixlen 64 scopeid 0x20<link>
ether 0e:c2:07:44:10:50 txqueuelen 0 (Ethernet)
RX packets 337 bytes 346443 (338.3 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 999 bytes 297424 (290.4 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlan0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether dc:a6:32:0a:ed:2b txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
Explaining overlays can best be told via visualization:
https://github.com/marcelbrouwers/swarmoverlayvisualizer
https://docs.docker.com/storage/storagedriver/overlayfs-driver/