iCloud storage is shared among all of your devices and isn't device-specific, so you can't "assign it" to one device. You have the option of not using it on your other devices. You pay for it via your Apple ID's payment method, so it doesn't matter whether you set that up via your Mac or your iPad.
There isn't an iCloud 'control panel' where you see all your devices then choose which one has access. You decide that by signing into iCloud on the individual devices you want to have access to it.
iCloud Photos works with the Photos app to keep your photos and videos securely stored in iCloud, and up to date on your iPhone, iPad, Mac, Apple Vision Pro, Apple TV, and iCloud.com.