For the first.
By default, containers are ephemeral. Any files written to the container's filesystem during runtime are lost on redeployments unless you

explicitly attach a Volume.

Volumes are separate storage spaces mounted to your service at runtime (e.g., / app/data), and persist files across container restarts and

deployments

Any data written outside a mounted volume vanishes when a new container is deployed or rebuilt.

For the second.
In-memory data (like objects in RAM or temporary caches) is tied to the running container process. If a deployment restarts the service (same container, hot reload), that memory can persist during that session. Key word can, sometimes it does sometimes it doesnt lol.

For the third.
Cached deployment happen when Railway detects no change in your build setup.

Full rebuilds are triggered by modifications to build context. Things like a package.json, Dockerfile, or some dep. And explicit rebuild commands/actions.

For the fourth.
Short answer no. Volumes only by design. Answered in the first.

Let me know if this was helpful!