EDIT: See @geoffaire 's post below… I was wrong about this.
Each focus can select which home screens are visible. It’s not possible to have a home screen that is only visible in one focus mode. Turn focus mode off and they’re all visible.
I’m hoping Apple develop the Focus modes - it would be even better to be able to limit the device to specific apps while in focus mode… as it is, swipe right and all the apps are there whether on the home screen or not.
You can choose for each focus mode which home screens are available from settings / Focus / choose focus / choose edit under customise screen (middle screen)
Where you do not select a focus mode, as default it will show all home screens, but if you go into Jiggle mode (while no focus mode is selected) and then tap on the dots at the bottom (above the dock) you can choose which screens show in default (no focus) mode.