Nextcloud is free and open source software that allows you to host your own file storage and sharing solution on the internet. As this is an open-source application you are free to install it on your server.

Yes, once your Nextcloud is installed, you are the only administrator on board. you have access to all the extensions available and compatible with your version.

Yes, you can and should update the Nextcloud version regularly. This can be done directly from the Nextcloud interface.