You need a compatible sized iCloud+ plan (There is also an Apple One Family plan, if you’re interested in the other Apple services included with it for you and your family), which would be either the 200gb or 2TB plans (If you have a lot of devices and photos, please just get 2TB), and then add them to the Family Sharing - Apple (UK) group, and then there will be an option for them in settings to join the plan with regards to storage. All files and backups [with family sharing] will be kept separate, but devices will be visible to each other in the Find My app, there will be a Family album in the Photos app (photos have to be intentionally added to it), a Family Calendar, the ability to share App Store purchases (tap on your profile picture in the upper right, tap purchased, tap a name), etc, by using Family Sharing.
Source: I’m on a 2TB iCloud+ Family plan.