When you order a Linux VPS you can pick an app to pre-install on first boot. The machine comes up with the app already running, so you skip the manual setup. It is optional: leave it on "None" for a plain OS.
On the VPS order page there is an App dropdown. Pick an app and order as normal. The install runs once, on the very first boot, on top of your normal account and network settings (those are never changed). It only applies to Linux; Windows VPS orders ignore it.
All of these are free, open-source projects. We install and start them; we do not modify them.
Apps that generate a password or database write a short text file in your home directory and in /root, for example oh-gitea-credentials.txt or oh-mariadb-credentials.txt. A marker file at /etc/oh-app-installed confirms the install finished.
For security, app interfaces and databases listen on the server itself, and only your dedicated SSH port is open to the internet by default. The simplest way in is an SSH tunnel: forward the app's port over your existing SSH connection, then open it on your own computer. For a web UI on port 3000:
ssh -p PORT -L 3000:127.0.0.1:3000 USER@IP
then browse to http://127.0.0.1:3000 locally. The same pattern works for any port (databases on 5432, 3306 or 6379; object storage on 9001; and so on). To expose an app to the public internet instead, put a domain with HTTPS in front of it (see the guides) or ask support about a dedicated IPv4 address.
The VPS is fully yours with root access, so you can install anything afterwards the normal way. The one-click list is just a convenience for the most common setups.
Straight answers.
No. One-click apps are free; you only pay for the VPS. The apps themselves are open-source.
No, the one-click apps are Linux only. A Windows VPS ignores the setting.
In a text file in your home directory and in /root, e.g. oh-gitea-credentials.txt. The file /etc/oh-app-installed confirms the install ran.
Yes. It is your machine with root access, so you can remove the app or install anything else at any time.