The Web Backend, Web Frontend, Submission Scoring Service, Code Execution Service (docker ikages). See also https://www.quora.com/What-is-the-technology-stack-used-in-K...
The lion's share of Kaggle's effort lies in their expertise in (1) making deals, and (2) making sure the challenge is legit (no bias in training data, not too easy, etc.). That's not easily replicable, so while an "Open-Source alternative" sounds good up front, the bulk of the work remains.