Unless you've got something sanitizing env prints in your logs, no, they don't get masked. I've leaked secrets accidentally while debugging environment variable issues.
Of course the bigger side of the issue is that anyone who can run code in the CI context can view all data in the CI context. So if an untrusted dev can write a build script that executes with secrets in the environment, they can leak those secrets. CI is RCE-as-a-service, you implicitly give permission for anyone who can edit build scripts to run arbitrary code. Like you say, they can echo the secret to a file & upload it elsewhere, and they can also use it directly from the edited build script.
Yes. I've done it. That's why there is an "approve and run" button when the PR comes from another repo. So that the maintainers can check if the PR is trying to exfiltrate secrets... or do other bad things of course.
The whole point of the github action secrets is that they are accessible by the CI when needed. Which means they are accessible. It's just a convenient way to store the secret in such a way that it is not completely public. But you still have to trust github and your maintainers.
