How does Android and ChromeOS being open source contribute to anyone wanting to create their competing platforms, and how much uptake have they gotten from mainstream users so far?
A lot. LineageOS, Amazon's Fire OS, OnePlus' OxygenOS, Android emulation layers and so on are only possible because Android is open source. It sets a low ceiling for any anticompetitive practices Google may want to pull off.
Nobody seems to really care too much about ChromeOS niche.
Mainstream users don't care about any of this and just want things to work. Open source allows other people to take something in a better direction when the original project stops "just working".
I do hope Firefox survives. I think it's really important for the web to have multiple implementations. But lets not pretend this is as bad as the IE situation.