Application-only authentication limits are applied on a per-app / ID basis, not per owner of the app. Note that app-only auth will only give you access to a limited number of read-only endpoints, whereas user auth will let apps post content etc as well.
This is not currently documented, but you are unlikely to have an issue unless you’re building a lot of bots. There’s a limit the creation of a small number of apps per day (this is also not documented as a specific limit, as it varies to prevent spammy usage of the platform). You should not use multiple apps / API keys to try to circumvent rate limits.
Modify/update would require a user context. In that sense, if one app posts content on behalf of a user, another app could come along and delete it, if it had been authenticated to that same user context.
So under a non-spamming context (it’s just to improve the UX of our football app, in the way users see twitter content), this means i can, for example, have 10 different bots (apps) under the same user, each of them with its own app rate limit, modifying the same collection, owned by the user who owns the ten apps.