What you should receive.
ACCESS
Domain registrar Hosting Site administration Any third-party service
DOCUMENTATION
What the site is built on What is installed and why Which plugins are paid, and what they cost Where backups go Anything unusual about the setup
TRAINING
How to update content How to add pages or products How to check orders or enquiries What not to touch
THAT LAST ONE
Genuinely useful. A short list of things that will break the site.
FILES
A copy of the site files and database.
Yours to keep.
WHAT TO ASK FOR IN WRITING
A handover document covering all of the above.
WHY
In a year, nobody remembers.
And if you change developer, that document is what makes it possible.
WHAT TO DO WITH IT
Store it with your business records, not only on the server.