Background of any PLC

Hai to all
This my first post.

Can anyone explain how the processor reads the PLC instructions

Yes.
It 'reads' the presence and absence of voltage at bits that make up registers that consist of either 16 or 32 transistors within a memory chip. (depending on the PLC) These are then put into another register where it is compared to yet another register. Depending on the returned information it makes a decision and then does the next register compare.
The exact details depend on the type of PLC and the instruction.
HTH

The PLC is a computer running a fixed program (the firmware in the CPU module). The ladder program is converted to bolean instructions and stored in memory.
The computer runs a set of tasks. The main three:
Read Input register. Solve bolean logic. Write to Output register.
This is the main part of a 'Scan'.
Other items are 'Housekeeping' (check for errors, low battery, etc.), communications (Program Port, HMI, etc.).

So, the simplified sequence is Housekeeping, read inputs, solve logic, update outputs, Housekeeping, etc.

Most PLC manufacturers explain this in more detail. Look for information on a 'Scan'.