You'll notice that it uses a lot of the unicode-range property, to split the font up into different languages, so not everything needs to be downloaded for every site (e.g. if your site doesn't have cyrillic characters), in addition to them serving WOFF2 as well:
In the downloaded archive, realistically you'll only have the TTF variation, like OpenSans-Regular.ttf, and will have to think about WOFF and WOFF2 conversions yourself. While those aren't too hard to do, splitting fonts up I still haven't figured out, so many of my sites with self hosted fonts have like 400 KB of arguably unnecessary overhead (I rather like the PT font family), which in some cases is the majority of the total download size.
I actually started building my own site where I'd keep some of the converted fonts that I personally like, but obviously it's not meant for greatness: https://fonts.kronis.dev/ (maybe some day I'll go back to it and actually figure out the font splitting)
Edit: in case anyone is curious about how to convert fonts, on Debian/Ubuntu I found these tools to be helpful: woff-tools woff2
Also Google still cares about serving the correct font version depending on your browser and OS, e.g. by not sending variable fonts (where one font file covers multiple font weights for example) to old browsers that didn't yet support them.
People self-hosting Montserrat are one bane of older browsers, because that seems to be a relatively popular font that got converted to a variable font file at some point, and which in browsers without variable font weight support then gets displayed at the thinnest possible font weight, which is really thin. (I concede that this is a niche complaint, but still…)
The tool parent and sibling comment listed allows you to do basic subsetting [1], and generates the formats you need. If you do this more often, it is of course useful to learn how to do this yourself. A commonly used tool is pyftsubset, part of fonttools [2]. I am not sure, but afair Google is running this under the hood.
I don't think it's quite the same, though! If you go to Open Sans, for example (https://fonts.google.com/specimen/Open+Sans?query=open+sans), and preview the CSS that it generates for the Regular style, then you'll get this: https://fonts.googleapis.com/css2?family=Open+Sans&display=s...
You'll notice that it uses a lot of the unicode-range property, to split the font up into different languages, so not everything needs to be downloaded for every site (e.g. if your site doesn't have cyrillic characters), in addition to them serving WOFF2 as well:
In the downloaded archive, realistically you'll only have the TTF variation, like OpenSans-Regular.ttf, and will have to think about WOFF and WOFF2 conversions yourself. While those aren't too hard to do, splitting fonts up I still haven't figured out, so many of my sites with self hosted fonts have like 400 KB of arguably unnecessary overhead (I rather like the PT font family), which in some cases is the majority of the total download size.I actually started building my own site where I'd keep some of the converted fonts that I personally like, but obviously it's not meant for greatness: https://fonts.kronis.dev/ (maybe some day I'll go back to it and actually figure out the font splitting)
Edit: in case anyone is curious about how to convert fonts, on Debian/Ubuntu I found these tools to be helpful: woff-tools woff2
I use them like so: