You are dealing with a chicken and egg problem. First you need to install an operating system (OS) on your Raspberry Pi. There are two obvious choices, Raspberry Pi OS and OctoPi (which is a customized version of Raspberry Pi OS - Lite). The full RPi OS will most likely have the drivers needed to interface with your 7" touchscreen but the Lite version of Raspberry Pi OS (i.e. OctoPi) will not. I'm guessing that the touchscreen has two connections to the RPi, an HDMI connection and a USB connection, so on either OS, the screen will display something, a desktop on the full OS and the text console on OctoPi.
If you install the full OS then your 7" touchscreen is happy but you don't have OctoPrint installed for you. There are, however, instructions on how to do that manually.
If you start with OctoPi, then there are instructions (a script) to install the full desktop but you may still need additional drivers to get touch working on the 7" touchscreen.
In either case, the OS needs a network connection to proceed. Instructions to configure a network connection are included in either OS install, but they will be slightly different.
To communicate with OctoPrint (and complete the installation) you will need a browser. This is usually on a separate system and, of course, connects through the network to the RPi running OctoPrint. Another useful communication protocol is SSH. The client you use will differ depending on the OS of your separate system, but if that separate system is Windows, then PuTTY is the recommended client.
OctoDash, OctoScreen, and/or TouchUI are all applications to designed to communicate with OctoPrint using a touchscreen. They all require a working OctoPrint.
My suggestion would be to start with OctoPi, get OctoPrint configured and working using a separate system. Maybe even 3D print something. Then using SSH / PuTTY, etc. install the desktop and any drivers needed to make your touchscreen work, and then pick an application to use.
If you decide to go the full RPi OS route and install OctoPrint manually, you will most likely need a keyboard. I have a 7" touchscreen but its not connected to my RPi running OctoPrint. On that RPi, in addition to a keyboard, I ended up attaching a mouse because my fat fingers on a tiny 7" screen didn't make for the most accurate pointing device. Eventually, I was able to use a bluetooth keyboard and bluetooth mouse which freed up 2 USB ports.
The people on this forum have more OctoPi / OctoPrint experise than 7" touchscreen expertise.