What does a Linux or UNIX managed host need so that Ansible can manage it?

No agent, but a Python interpreter for most modules (the deck names Python 2.6+ or 3.5+), and on SELinux systems the python3-libselinux library.

• Network access for the control node, normally SSH.
• Python, because most modules are Python programs executed on the host. The classic minimum is Python 2 (2.6+) or Python 3 (3.5+). Newer ansible-core releases have raised it a lot (current releases need Python 3.9 or later on managed nodes), so check the support matrix for the version you run.
• On RHEL 8, the platform Python package is usually enough, or you can enable the python36 (or python27) application stream.
• With SELinux enabled, install python3-libselinux so file modules can set SELinux contexts correctly.

What the host needs depends on how the control node connects and which modules it runs.

Go deeper:

• Installing Ansible: node requirements (official docs) — current managed node requirements (Python to run the generated module code).
• Releases and maintenance (official docs) — the support matrix of Python versions per ansible-core release for control and target nodes.