It's nice that you found your way here, we hope that you are able to find some interesting repositories which will help you!
Greev.eu is a Minecraft Server that opened by @Alex1607 in early 2015 and is the primary project of him. Since around 2016, we have been developing our own plugins, website, moderation bots, and more, and are proud to do so. At the beginning, we started with only one developer (the founder) and later found two additional developers.
At the time of writing, Greev has around 70 to 80 daily users and over 600,000 registered users. It is still actively maintained and worked on a daily basis by numerous volunteers.
We have over 100 Repositories on our GitHub account, from which only a tiny fraction is open sourced. Which doesn't mean we don't value open source! It's just that we are in a position where some projects contain too many interactions with internal systems where it just doesn't make sense to publish the source code. Nevertheless, we are working hard on removing these and publish these projects then.
By now, we already published a handful of projects which we either forked and modified for our use case or created on our own. You are welcome to use them to create our own server, create other systems with them, or even collaborate with us on them.