Virtually all of this map was hand-drawn in ProCreate on an iPad 3 using an Adonit Touch stylus. There are a few "pattern brushes" in that app which help with things like the the jungle trees and the ocean waves, but everything else is just drawn. The shape of the island and mountains isn't based on anything other than doodling with the idea of a vaguely volcanic tropical island in mind.
The cloister overhead plan was laid out in Adobe Illustrator. It's far larger, sharper, and more detailed in the original file, with many upper floors and basement dungeon levels. The "3D" isometric extrusion of the cloister is actually just faux-3D, a technique I use in Illustrator where I take the overhead plan and rotate, squash, duplicate and move by a certain amount, and then blend. I took a screen shot of that and traced over it in ProCreate.
The last step was to bring it all into Adobe Photoshop, create a "parchment background", lay out the various pieces, and add in the text elements. Then I did save-for-web and picked settings that looked good but kept the resulting file under 2 MB.
The cloister itself is about 300 feet on one axis, making the hypotenuse around 500 feet. Since you can line up 30 of the little cloister images end to end and have them stretch from one side of the island to the other, the island winds up being about 3 miles across.