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If I run two mysql containers, it won't necessarily take twice the resources of a single mysql containers

It's complicated, but essentially, no.

Docker images, are built in layers. Each layer is a step in the build process. Layers that are identical, are shared between containers to the point of it taking up the ram of only running the layer once.

Although, it should be noted that docker doesn't load the whole container into memory, like a normal linux os. Unused stuff will just sit on your disk, just like normal. So rather, binaries or libraries loaded twice via two docker containers will only use up the ram of one instance. This is similar to how shared libraries reduce ram usage.

Docker only has these features, deduplication, if you are using overlayfs or aufs, but I think overlayfs is the default.

https://moonpiedumplings.github.io/projects/setting-up-kasm/#turns-out-memory-deduplication-is-on-by-default-for-docker-containers

Should you run more than one database container? Well I dunno how mysql scales. If there is performance benefit from having only one mysqld instance, then it's probably worth it. Like, if mysql uses up that much ram regardless of what databases you have loaded in a way that can't be deduplicated, then you'd definitely see a benefit from a single container.

What if your services need different database versions, or even software? Then different database containers is probably better.

it won't necessarily take twice the resources of a single mysql container

It will as far as runtime resources

You can (and should) just use the one MySQL container for all your applications. Set up a different database/schema for each container

AFAIK it won't and should you still get a bottleneck you can limit the maximum resources a service may use.

A compose file is just the configuration of one or many containers. The container is downloaded from the chosen registry and pretty much does not get touched.

A compose file 'composes' multiple containers together. Thats where the name comes from.

When you run multiple databases then those run parallel. So every database has its own processes. You can even see them on the host system by running something like top or htop. The container images themself can get deduplicated that means that container images that contain the same layer just use the already downloaded files from that layer. A layer is nothing else as multiple files bundled. For example you can choose a 'ubuntu layer' for the base of your container image and every container that you want to download using that same layer will just simply use those files on creation time. But that basically does not matter. We are talking about a few 10th or 100th of MB in extreme cases.

But important, thoses files are just shared statically and changing a file in one container does not affect the other. Every container has its own isolated filesystem.

I understand the architecture, I'm just not sure about how docker streamlines separate containers running the same process (eg, mysql).

Quite simple actually. It gives every container its own environment thats to namespacing. Every process thinks (more or less) it is running on its own machine.

There are quite simple docker implementations with just a couple of hundreds lines of code.

Nah, that's not how it works ! I have over 10 applications and half of them have databases, and that's the prime objective of containers ! Less resource intensive and easier to deploy on low end machines. If I had to deploy 10 VMs for my 10 applications, my computer would not be able to handle it !

I have no idea how it works underneath, that's a more technical question on how container engines work. But if you searx it or ask chatGPT (if you use this kind of tool) i'm sure you will find out how it works :).

Separate. That's the whole point of containerisation! Otherwise you're just doing a regular deploy with extra steps