[belorn]

The first aspect is that every* TLD has it own registry and system. For the generic ones you got ICANN accreditation process, but there is also a bunch of registrar reseller that act as a middle man between ICANN and other registrars.

Usually most processes involve some form of capital investment and/or technical capability. Country specific TLD can either be easier or much much harder depending on which country.

* Not really everyone.