Raspberry Pi as a UDP or TCP server

Ok, here is the idea. I want to use my Raspberry Pi as a server between PC application and microcontroller. Microcontroller unit will do some stuff and communicate with raspberry Pi via UART and Raspberry Pi will comunicate with PC application via TCP/IP or UDP protocol (Accesible over WWW). Also, I want my raspberry Pi to act as a interface between human and microcontroller (LCD and buttons will be connected on raspberry, so that microcontroller is not loaded with user interface and can do his job peacefully ).
My problem is, that I don't have basic idea how stuff works on Raspberry Pi. How can I run multiple programs on raspberry (working with LCD, buttons, TCP/IP, UART with mcu)? How does even programming look like on raspberry? I am used to programming on low level with mcu-s, but how can some device like pc (raspberry) works with low level stuff? And all low-level stuff must be mixed with high-level programming for TCP/IP communication and all that. Where can I start to get the basic idea how this stuff works?
PS.: I will probably write my PC application in QT, because I want it to be cross-platform but this doesn't really matters now.

These might you get started

https://www.linuxjournal.com/content/raspberry-pi-perfect-home-server
**broken link removed**
https://www.raspberrypi.org/forums/viewforum.php?f=36

Not really. This doesn't cover my questions.

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I think I am getting the picture here. I can use wiringPi for low-level stuff (lcd on raspberry pi and UART with microcontroller) and then I can even use qt for high level programing like TCP/IP stuff. But can I run all these programs simultaniously like multitasking in background?

Not really. This doesn't cover my questions.

Click to expand...

Respectively you don't yet understand how Linux operation system is related to the answer. Perhaps you should read about it though.