The order of the discs shown in the showcase screenshots are in this order;

-13
-cat
-blocks
-chirp
-far
-mall
-mellohi
-stal
-strad
-ward
-11
-wait
-otherside
-5
-pigstep
-relic

It's the same order as in the creative menu

The 1.21 discs in the "I'm not making showcase builds for these" screenshot:



-Creator

-Precipice

-Creator (music box)



I just threw them in there because I'm lazy

Mods were used to decorate the showcase screenshots.

Most showcases used the Architect's Palette mod
I actually started to rely on this one because I ran out of ideas for vanilla blocks to use...
https://www.curseforge.com/minecraft/mc-mods/architects-palette

The showcase for Wait used portals from the Aether mod
https://www.curseforge.com/minecraft/mc-mods/aether