@Poncho
you have to go in Website Tools > Code Injection and use the first box to insert inside the <head> tags the code to embed your fonts.
E.g. for Montserrat, via Google Fonts, I used this code:
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Montserrat:ital,wght@0,100..900;1,100..900&display=swap" rel="stylesheet">
to load the full axis of width.
Then you can use inside CSS. E.g.
h1 {
font-weight: 300;
}
It should be useful just if you upload image with the very same name and this replace the one in the library. Now what happens is something "weird": you can have more images with the same name (it is possible 'cause it's in a different path inside Squarespace's CDN).