I personally just throw them at the top of my files as long block-comments, or sometimes inside/around very heavy functions. For example i often add little diagrams for when dealing with some bit-fiddly logic parts to easier visualize the bit-layouts.
But for architecture, either a whole text-file for it or at the top of the module
Thanks! Do you deal with the logic getting split/shared around the code ? For instance on the credit card example there will be probably be one central class (the transaction class?) but you'd need to know the whole logic in the card registration part or the webhooks as well. I guess you don't stick a diagram everywhere ?
