> Ok, so then it's like a package manager manager.

Still no. asdf manages the versions of the runtimes themselves. E.g. I have a project that uses ruby 2.7.2 and Terraform 1.1.7. If I'm using asdf, I declare this in the .tool-versions file of the project, and then when I navigate to that directory, every invocation of `ruby` or `terraform` will run those exact versions.

Separately, packages (Ruby Gems, Python packages, etc) will also be isolated per-version of each runtime, but that's a side effect rather than the goal with asdf.

Actually no, it manages the versions of the languages themselves, but doesn't interact with their package managers at all. The .tool-versions file it uses to mark the current version could be thought of as a lock file as well. It replaces (for Ruby) rbenv/rvm, not rubygems. Rustup, not Cargo, etc.