Greenhouse and Lever are good but are there any open source alternatives? I need one for my company but I don't want to spend the exorbitant amount and I want some customizations.
The existing options don't seem anywhere as good as existing closed-source software.
As was pointed out, you haven't built one. More particularly you need to create software that allows a community to support, use and profit from it. For example, could you make it so that niche industry build one and pay for a low support contract or even run it on their own like a Discord server?
Each application was an issue. It contained the résumé and information about the applicant. We had an issue template.
Stages of the process were labels. We'd move the issue around and our discussions would be in comments on the issue. It worked for us really well.
We used GitLab for administration as well.