By default the app is just a terminal with an on-screen keyboard. Packages installed and run through it are normally terminal only, although there is some amount of support for connecting to a separate app that runs an x server.

There are several other projects that download arm Linux distributions, and install them inside termux to get a full desktop going though. Normally for these you actually interact with the desktop through a VNC client connected to a localhost VNC running on the Linux distro. A VNC client is a good way to handle accessing the desktop since most of them come with a lot of UI design/polish for controlling a desktop computer via a phone screen.