Thanks for your question, Momo! 🙌🏼
Milestones are predetermined timelocks.
It is for the founder/project creator to share regular project updates using nostr, in line with the milestone dates which were set while creating the project.
There are no third parties - only the project creator (founder) and the investor(s).
Feel free to take a look at the source code: https://github.com/block-core/angor