On that Centennial Medal that looks like it started with pellets of metal--it probably was. The process is called Powder Metallurgy, and is a fairly common but specialized metallurgical process. Here is the introduction from the article on that process as presented in Wikipedia, followed by the web link. The article is pretty good at explaining the whole process, so no need for me to re-type it.

It is possible that the medal was formed directly by a powder metallurgy process, or that the blank planchet was formed by powder metallurgy and then the planchet was struck in the normal way. Close examination would reveal the approach used. If you want a more detailed technical description, see: en.wikipedia.org/wiki/Powder_metallurgy

To read the earlier E-Sylum article, see:QUERY: HOW WAS THIS CENTENNIAL EXHIBITION AWARD MEDAL MADE?(www.coinbooks.org/esylum_v15n42a14.html)