What moving off Squarespace actually involves
Squarespace exports a partial WordPress XML file: blog posts and basic pages come out, but galleries, product pages, index sections, and most of the layout do not. So the content transfers and the design gets rebuilt, which is generally what people want anyway, since the reason for moving is usually that the template stopped fitting.
Everything you have written is inventoried first, including the pages the export misses. Images come across at full resolution rather than the resized versions Squarespace serves, so photography that has quietly been getting softer over the years is restored.
The redirect work matters here because Squarespace URLs are distinctive. Old paths are mapped one-to-one, and the Squarespace-specific patterns are handled explicitly so search engines follow you rather than dropping the pages they already rank.
If you are on Squarespace Commerce, products and categories are rebuilt with their URLs preserved. Four to six weeks is typical. Your existing site keeps running until you approve the replacement, and nothing is canceled before then.
One thing worth checking before you start: whether your Squarespace plan is annual and when it renews. There is no refund for the unused portion, so timing the move for the end of a billing year rather than the start of one keeps a couple of hundred dollars in your pocket for no extra effort.