1 - Rocket.Chat is the world’s largest open source communications platform. You can install it on your own server, and use any Browser, our Desktop App and Mobile App to access your own server. It can be both used to employee communication and customer care (Omnichannel). For this, you can integrate with Instagram Direct, Telegram, Whatsapp, Email, Website and other way to connect the “exterior world” on it.
3 - Yes. If you are talking about Authentication, it supports a lot of them, like LDAP, OAUTH, etc. If you are talking about consulting other databases, you can also do that in a lot of different ways. You can have some incoming webhooks, or outgoing webhooks integration, or you can write an App that does more things: we have a whole App Engine inside Rocket.Chat for that
4 - Rocket.Chat uses Mongodb as it’s main Database.