That seems RG Offset Transfers to me which can be stored & may be printed on demand, second did you mark most of the stuff is typically graphic in nature with some trippy pictures & number of different colors that also confirm that it is a transfer job not a direct screen printing style except few font based articles.
about question 3. may be they are using some different physical tee shirts body pics to swap artworks on to different colors in different styles & showing the variety of colors online cant rule out this chance, so when they do receive specific order in specific color they just take out the blank & prepare the transfer according to the demand respectively.