Take a look at the code in the tinkerbell/tink GitHub repository.
Tink Worker is an agent that runs in the operating system installation environment. It is responsible for retrieving workflow actions from Tink Server and executing them. Actions are published as Docker containers with Tink Worker acting as their launcher. When an action finishes, Tink Worker reports the status to Tink Server
How Tink Worker starts
Tink Worker's boot process does not follow any particular rule and it is left to the operating system installation environment.
In Hook, Tinkerbell's default operating system installation environment, Tink Worker is launched by a multi-staged process. A small program called hook-bootkit is launched as a serivce, reads the Kernel command line for Tink Worker specific parameters, then launches Tink Worker as a Docker container. See the LinuxKit configuration file and documentation for how hook-bootkit is launched.