When you have your website created by an agency or by a freelance developer, it is up to you to pay particular attention to the general terms and conditions of sale and more particularly to the ownership of the data present on your site.
There are actually several solutions:
1) You choose an open source CMS that can be easily installed on any type of server. In this case you can own the source code and simply keep 100% control over it.
2) You choose a "home-made" CMS in this case the provider who designed the site will most certainly have control over the backoffice that you will not be able to retrieve later.
3) Finally, new solutions have appeared in recent years such as Wix, Squarespace or Webflow. The CMS is available online, you pay a monthly access fee, the platform takes care of the updates and you own the "front" (what the client sees) but not the back. It is often not a problem (not to own the back) because these platforms allow you to evolve with the codes and technologies of the Web and guarantee you a website that works over time and on all media (mobiles, tablets, etc.).