Question about Docker IPs

[Korval]

I'm a novice at networking, so apologies if this should be obvious. I'm just learning as I go. I've recently set up my first Terramaster NAS and have finished getting the most important file-access stuff going. Then I decided to try my hand at learning basic Docker stuff.

I have one simple container working (for my e-reader) and had just started looking at the post-installation steps for an immich server. I'm following the guide at https://docs.immich.app/install/synology (the Synology guide seemed to match Terramaster pretty well). It suggested noting the container's IP and using it accessing the web app. As far as I can tell, the IP addresses shown in the container manager are bogus. For both of the containers that I tried, web access works with either my internal IP (i.e., 192.168.0.x) or my current external IP--after forwarding the appropriate port, of course. Am I missing something? Are the containers' IP addresses "virtual" like the IP address shown in the TNAS's Control Panel under "Remote Access" (and therefore seemingly useless)?