In terms of login - you are logging into whichever Designer plan (not site) you have - so Free (Starter), Lite, or Pro, or Team.
The only way I can see to limit to just that one site (without venturing into the Team plan options) is to have a separate (from your own plan) Designer plan for you and the freelancer to use and log into.
Two ways to do that:
Starter plan (free with limits - add CMS hosting to unlock limits)
Lite plan (same price as CMS hosting for one site - but gives you 10 projects to work on together - then add hosting on top, when ready)
There is still the limit that only one of you can be logged in at one time though…
Hope that helps