Old standard communication way between two devices, there are several ways it has implemented.
Logically very easy to connect, CH341A RX,TX to SIM800L TX,RX (RX to TX and TX to RX). The only problem is that the maximum voltage level of SIM800L UART is 3.3V (I chose 2.8V) and CH341A works with 5V by default, so I have to use a level shifter (e.g. TXS0102). As SIM800L recommended voltage for power supply is 3.4V-4.4V, you should power it from a 3.7V battery, not directly from 5V of USB. Another reason to use battery is that USB current is only 300mA-500mA and SIM800L can use 2A.
After the good connection I could access it under /dev/ttyUSBx in my Linux. SIM800L works like old analogue modems, You can manage it with AT commands in a serial monitor. You can choose several programs: screen, putty, ... I used PUTTY now. (It works similar in Windows, just select correct COM port instead of /dev/ttyUSB0.)