I join my friends created a portal for zoom meetings and social groups. It includes analytics, push-notifications, real-time database (with firebase), and web share API. In mobile, I also added a small feature that scrolls to the meetings view if the user's viewport does not include it.
I created the web platform from scratch. At first, I use create-react-app and followed the android design with react-material, but then as we got feedback from users, I designed it myself and switch to css modules. The migration was gradual and I did not convert all the app, but the react-material stuff lazy-load on pages that needed it. I also integrate react concurrent mode (the search feature is using useDeferredValue). After a while, I convert it to next.js. I also helped the other clients (iOS and Android) with the firebase functions.
Some of the challenging stuff we deal with is banning from the google play store (because users create meetings with the word CoronaVirus) - so we had to add the option to auto-block those meetings. Following apple's app store rules - we had to allow manage complex user registration and allow ban users. We did not have a designer/product - and we have to figure out how to design the app and what features will it include. We did the marketing ourselves in a Facebook group. We also had to deal with competitors and support our users. So the whole experience was much more intense than a typical startup company.
