I am working with Adobe Illustrator and Photoshop and I prefer to work in fullscreen mode in a separate Desktop as is often used with working on MacOS but when I want to create a new Desktop I cant do this because instead of FullScreen icon I have '+' icon that allows me only to fill main desktop without hiding menu and dock.
Adobe apps don't use the Mac-standard fullscreen.
You can put it into fullscreen from the View menu:
Click any image for full size
…& add key commands if you wish:
…but because it still isn't the same as the Mac-standard fullscreen, it will not create its own Space/Desktop as you do this.
Best is to assign it its own Space beforehand, so it doesn't sit above other apps when you fullscreen it.
Generate a new Space in Mission Control, then switch to it before launching Photoshop/Illustrator.
Right click in the Dock & set to This Desktop.