Does SoloLearn use SQL or NOSQL database ?
SoloLearn does not expose details about their technology stack to the public, so there is no way to know this for sure. Maybe they use both, for different purposes. SL developer don't read these forums and even if they were, they might not want to answer this question in any case.
Why wouldn't they want to share such info. Facebook engineers have a lot of posts on the internet talking about the technologies they used. I don't think there is any problem with sharing that. I mean, the app is about learning and knowing such things, could help people decide what stack they want to learn or use to build their apps.