If so, that implies a massive already-existing network, which is great. If not, then it may take a while for most devices to get onto iOS 14.5, and delay the usefulness of the tags.
Yes they will. I'm on iOS 14.4.2 and I already see the items section in FindMy. 14.5 is required to add AirTags to your account, but they're using the same technology that Apple added in iOS 13 for things like tracking AirPods.