Do we even do terrain here? Well I put some minis on it so I am pretty sure this still counts. ;) I finally finished a large chunk of the terrain I had begun a few months ago so I put it on the table to see some possible layouts.
All terrain is crafted from scratch from XPS foam, sand, toothpicks, some bits, tufts and moss. Heavily inspired by Black Magic Craft on YouTube (namely modular ruins, dungeon tiles, pillars, dungeon stackers). The acid pools and spike traps I came up with myself.
Cutting bricks for ruins can be done pretty decently with a sharp kitchen knife or an olfa knife for example. You could also get a handheld hotwire cutter and fix it in a vice or with clamps and use a straight piece of wood for a guide. Kind of like a makeshift hotwire table. I did both things in the past and they work well enough for small to medium sized projects.
Truth is nothing beats a hotwire table for efficiency when cutting looooots of bricks. So if you want to craft a lot of brickwork you'd be very happy to have a table still.
I've always been a little leery of xps foam. Looking at your stone ruins makes me want to give it a go.