brent Your pics in the other thread are nice.
Thanks for the kind words, but there isn't any secret here. It doesn't take special skill with a screen capture utility. It's something everyone can do. If you just start a VM in the host window's client area, and then maximize the VM's window, both operating systems are available at the same time. The VM is available by clicking on it, and the bottom panel of the host is still available at the bottom. For example, here's a Solus 4.4 Budgie VM running inside the client area of the Solus 4.4 Plasma host machine's window.
With one mouse click on the Budgie window, I can change to that operating system, and similarly, with one mouse click I can change to the host's Plasma operating system. Compare that to a dual-boot scheme, where you need to save everything in the current operating system, shut it down, and then launch the other operating system, with neither of them giving you quick access to the other.
But the most important thing, in this context, is that by having both operating systems available simultaneously, that means that you can operate the child OS (such as to open a menu) and then change to the host operating system (just press CTRL+H) to use its screen capture utility (Spectacle, in this case) and capture an image on the client OS. If you use the screen capture utility on the clent instead, of course that menu that you want to capture will close when you click to select the rectangular region you want.
So, taking these tightly-focused pictures doesn't require any special skill at all, it just requires that you work with a VM and your host computer at the same time. I can help you (and anyone else who wants to know) to use VirtualBox to create virtual machines. Just let me know, and I'll type up another tutorial for installing and using VirtualBox 7. It's not only free, it's in the Solus repositories. You can get it as easily as this.
Creating and using VMs doesn't require any occult skills that you need to learn. You're already more than sufficiently skilled at using such a program. You just need to decide it's what you want.