I did not find anything about their software. Is it open source? Is there an SDK? Can I use WhatsApp / Telegram / Signal? How can I develop my own apps? For m

I did not find anything about their software. Is it open source? Is there an SDK? Can I use WhatsApp / Telegram / Signal? How can I develop my own apps? For me this is currently a hard no, because even though I like the UX, I will not buy a closed software product, as even Android is modable.