Okay. So you need to build at least one page, and host it somewhere.
To create the page, you’ll need some HTML/CSS code (and maybe some Javascript). If you want that Squarespace-like drag-and-drop experience, you’ll need some kind of editor to write the code for you. See if any of these seem like what you want. I haven’t used any personally (I’m pretty comfortable writing HTML/CSS by hand), but I’d try BlueGriffon or Sea Monkey.
Then for hosting, you have lots of options. Some are free. Check out this list to get an idea. Amazon AWS and some of the others aren’t exactly “FOSS,” but GitHub/GitLab Pages are pretty harmonious with open source software, and most of the hosts listed won’t bloat your site with creepy analytics and stuff.
Finally, if you wanted a custom domain (e.g. “magneticfusion.com” instead of “magneticfusion.github.io”), you’d need to find a domain name registration service and point it at your hosted page. This part would not be free.
I don’t think any “one stop shop” solution as simple and foolproof as Squarespace exists in the FOSS space, although I’d be happy to be proven wrong. As is often the case with FOSS, you need to do some tinkering and cobbling together yourself. But it’s not too hard, and you’ll learn some neat stuff in the process.
Let me know if you have any follow-up questions, I’m happy to help.