You’ll find many posts on what it is and how to Flutter, perhaps a critique of using Dart or Flutter from a certain perspective. I recommend reading them first if you want a better idea of what it is, or how to code in it.
tl;dr if you’re thinking of using a “free website builder”, don’t. Pay someone to build you a good static site.
If you know someone who’s think of using one, tell them, no, and offer to build it for them if you can, or suggest someone.