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Kitamura explains the effective top level domain (eTLD) but it's useful to understand the fragility, construction, and management of the list of public domains, that is, the part of the fully qualified domain name (FQDN) that is controlled by a registrar. These fragments act like the literal top level domain in that the part of the FQDN to the left of the effective top level domain identifies the authoritative party. For example, .com is a classic TLD but .co.uk functions in the same way.
To understand the governance model, see this blog post and the referenced report.
M3AAWG. “The Present and Future of the Public Suffix List.” Messaging, Malware and Mobile Anti-Abuse Working Group, May 23, 2023.