What components or resources, besides files, can be isolated inside a container?

In a container, not only files and the filesystem are isolated, but also several other components and resources:

• Processes — containers have their own process space, isolated from the host and other containers.
• Network interfaces — containers can have their own network namespaces, IP addresses, and ports.
• Users and groups — isolation by user identifiers (UID/GID).
• IPC namespace — isolation of inter-process communication.
• UTS namespace — isolation of hostname and domain name.
• Kernel resources (cgroups) — limits on CPU, memory, disk I/O.

Thus, a container provides OS-level isolation, creating a separate environment for running applications.