That constraint shapes what gets built. A site you can update yourself matters more than one with a slightly better animation you would need help to change. Tools are chosen to match your actual comfort level rather than what would be most capable in expert hands. Anything requiring specialist knowledge to make a routine change is the wrong choice, however impressive the result, because the first time you need a price updated on a Friday afternoon you will discover what that decision cost.
The handover is built during the work rather than assembled at the end. Documentation gets written as decisions are made, which is when the reasoning is still fresh and specific. You get walkthroughs of the things you will actually do: updating content, reading what your analytics is telling you, adding a page, checking whether something is working. And you have access from the beginning rather than receiving it at the end, so you are already operating the system before anybody calls it a handover.
The measure of whether it worked is straightforward. Somebody asks you to change a price, add a service, or explain where your customers come from, and you do it without contacting anybody. If a handover requires a long meeting to explain, the work was not documented well enough, and that is a failure on the provider side rather than a gap in your ability.
There are limits worth naming honestly. Some things genuinely require expertise, and rebuilding a site or restructuring an advertising account is not a task anybody should expect to do themselves. What you should be able to do is everything routine, plus enough understanding to evaluate a proposal from whoever you hire next.
The documentation matters more than the training session, because a walkthrough is forgotten within weeks and a written procedure is available when you actually need it. Short instructions for the specific tasks you will perform, written for somebody who does not remember the conversation, are what make a system operable six months later. If the only record of how something works is a recording of a call, the knowledge will not survive.
There is a reasonable middle position worth naming, which is being able to do everything routine while retaining somebody for the occasional larger change. Updating content, adding a page, and reading your numbers should be yours. A significant restructure, a new integration, or something that has broken in a way you cannot diagnose is reasonable to outsource. The distinction that matters is whether you could hire anybody to do it, rather than being dependent on one specific person.