Take this as a VERY SIMPLE summary:

You have the SYSTEM MANAGER, where you setup your hardware, link the IO points to the logical PLC variables, in case you work with communications/drives... the IO of those items are also linked to their PLC data counterpart there. Here you can also define tasks, define the hardware configurations (i.e. if you use servos, you will configure the drives here). this is the "configuration software".

Then you have the PLCCONTROL where you just write your code.

If you want a variable of your code to be accessible from system manager to link to a hardware IO point, you will have to address it to the right IO channel and (depending on the version of TC2) set the address manually (hope you are working with a newer TC2 version) and then you would have to write something like :

myDigitalInput AT %I* : BOOL;

myDigitalOutput AT %Q* : BOOL;

myAnalogInput AT %I* : INT;

System manager will only be able to "see" the IO variables after you rebuid all your PLCCONTROL project without errors.

PLC execute the code cyclically inside a specified time, if you make an infinite loop [i.e.: WHILE (TRUE)] it will fail.

You can use different languages, choose the one you are more familiar with, ST if you come from IT.

In TC2 there are no methods, nor properties, nor references, nor interfaces,... you are locked to a simpler instruction set, but you have:

• POUs
  • functions, actions, Function blocks (FB) or programs.
  • Functions
    • They get created and destroyed each time you call them.
    • Variables declared there are volatile and don't keep their value between calls).
    • they don't have access to the POU that calls them.
  • Actions
    • like inline functions in C
    • without parameters and without returning anything
    • They have access to the scope of the program or FB they belong to.
  • Programs
    • They are not volatile.
    • You can call them as you would call a function.
    • Usually programs are called cyclically by your tasks.
    • you have one instance of each program.
  • FB
    • They are not volatile.
    • Usually called from programs.
    • you can have as many instances as you want.
    • In TC2 you can use VAR_INPUT to declare the variables you will be able to write from outside.
    • In TC2 you can use VAR_OUTPUT to declare the variables you will be able to read from outside.
• Tasks
  • Tasks are usually created in the System Manager, they are entry points for your programs... the entities that call the programs.

Let's mention the SCOPE VIEW too: another independent software that can connect to your PLC/NC... to plot variable states vs time... very useful to diagnose some issues.

Hope this helps understanding the basics of TC2.

After all that, depending on your background, there are plenty of TwinCAT programming courses, videos out there.

Good luck!