[chromedev]

You can find the releases here, and if there is a delay it is only a few days at most:

https://github.com/Telegram-FOSS-Team/Telegram-FOSS/releases

Only the clients are open source, but anyone could use the source code to create their own network.

[saagarjha]

This repository has been notoriously bad at doing this: https://github.com/overtake/TelegramSwift

[maqp]

The clients wouldn't feature E2EE for groups even if you'd write your own server from scratch so no, nobody should use Telegram for anything.

[literallycancer]

Without server source? Might as well start from scratch and use something like matrix.

[chromedev]

Matrix doesn't compare at all and Element is their client to promote their SaaS solution.