Yes! If you already have a Squarespace website, but want to switch to a new platform, you can do this. Not all your content will be transferred, but you can export your Squarespace website into an .xml file (this is particularly useful for exporting to WordPress).
Website builders can be tricky to move between because so much of their content is reliant on their unique platform’s code. Some make it easier to switch than others, so always check before starting. In many cases you’ll need to start again from scratch, which is less than ideal, but you may be able to use a third party migration tool such as Cart2Cart (for online stores).