A little while ago I noticed that SPC TAB was no longer respecting layouts in Spacemacs. This feature was part of develop but has since been removed it seems. There is a separate branch that has this code and it has been updated to require the spacemacs-layouts layer variable spacemacs-layouts-restrict-spc-tab to be set to true, t. Not sure if there is a bug with this code, or no one had chance to merge it to develop https://github.com/syl20bnr/spacemacs/tree/0e9866efcee9915f3418cec11371d02ee619ced2/layers/%2Bspacemacs/spacemacs-layouts

Ah, correction, its only the documentation that isn't included in develop (I will do a PR). To enable SPC TAB to only switch between buffers in the same layout, add the spacemacs-layouts variable to the .dotspacemacs-configuration-layers

(spacemacs-layouts :variables spacemacs-layouts-restrict-spc-tab t)

I have created the PR https://github.com/syl20bnr/spacemacs/pull/13124 to add documentation to the layer, showing how to configure the restricted functions. I have also added these details to the Practicalli Spacemacs book https://practicalli.github.io/spacemacs/working-with-projects/layouts.html

I am pushing more videos to the Practicalli Spacemacs book at the moment, if you have any suggestions for videos you would find useful, please let me know.