If you’re looking for large projects maintained by large numbers of people all open-source, the Massachusetts Bay Transportation Authority (the city of Boston’s transit authority) is all open-source:

if you’re looking for real use cases of application you can start with some opensource ones:
Bytepack was a product by Dashbit(Jose Valim’s company) to help people sell payed versions of libraries(like oban_pro/oban_web for example).
Changelog.com is a network of software related podcasts, their website/podcast feed is written in elixir and is opensource.
LiveBeats is a demo app for online radio written with phoenix live view.
Plausible is an analytics tool, alternative to google analytics.

social networks with activity pub

Akkoma is a microblogging federated social network using activity pub, like mastodon. written in elixir.
BonfireNetwork is also a federated social network using activity pub, iirc it uses live view. it’s not restricted to microblogging there are some other usages.
Mobilizon is a federated events social network, using activity pub. kinda like facebook events or meetup, but open source and federated.

The question is: what are you interested in?

There are some codebases (for example ecto, phoenix) where it will be extremely hard to understand what is going on as a beginner (or even as someone more experienced ) as they are heavily using metaprogramming.

If you are interested in web, you can take a look at one of my projects that we never managed to finish: SSL MOON / SSL MOON · GitLab . It was developed by beginners, so the concepts there should be pretty easy to understand as long as you read how core technologies like ecto, phoneix, liveview work.