Are you asking how wannabee shantymen learned to sing shanties (performance), or how an experienced shantyman added to his repertoire (sources of the songs)?
I expect you probably are looking for literary references to any of the above.
Hugill certainly learned a lot of his songs from older shantymen, the primary sources, and then from his own literary research and from the research of others. And he learned how to lead shanties from his primary sources as I would reasonably expect to be true of other aspiring shantymen in the 19th century.
With regard to the sources of traditional shanties, I expect that some were created aboard ship by individual shantymen. Others were adapted from shore songs such as broadsides or music hall songs, or field songs or other work songs. And there is clear evidence of shantymen cobbling together a new shanty from the verses of older shanties.
I doubt if any shantyman ever received royalties, or payment for product placement.