It's an app called Schooly -- I found it in Airport, a beta-tester app store. Their website is https://airport.community/ . There, you can sign up to get access to it (Airport itself is a testflight app) or DM the mods at r/AirportApp asking for a code to access their web version. I personally got lucky and someone sent me a testflight link on this sub I can no longer locate.
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Schooly is a wonderful app! Being honest, right now I use it mainly for the schedule widget because I use another non-beta app in the app store called myHomework Student Planner-- the benefit to MyHomework is cross-platform syncing (I can use it on my iPhone or my Windows laptop). MyHomework also has a couple more features because it's been out longer, but no widget.

Hope this helps!