I noticed in several Facebook Groups that I follow that a lot of newer Web Developers are asking questions about what to include in the service of maintaining a website for a customer.
When I first started offering web development and maintain site tasks for web customers I developed this check sheet of tasks. I used to use this foundational checklist and had one out for each site I managed. I would use this list each week, month and year to be sure that I completed each task for each site. Later on I made it a digital version and added the date to each item.
Sharing a PDF copy of items that should be done on a daily, weekly, monthly, and annual basis.
Of course it will all depend on the host being used, and the terms of the contract for providing this service to customers.
Maintenance-Tasks-Checklist (PDF format).