Which reagent atoms replace the atoms of another reagent in the substitution reaction? Explain with an example.

Substitution reaction – a reaction between a simple substance and a complex? as a result of which the atoms of SIMPLE SUBSTANCE replace the atoms of ONE OF THE ELEMENTS IN THE COMPLEX. As a result of such a reaction, a new complex and new simple substance is formed.
For instance:
2 K + 2 H2O = 2 KOH + H2 (K replaces the hydrogen atoms in the water molecule and a complex substance is formed potassium hydroxide and a simple one – hydrogen gas).
Sometimes, after a substitution reaction, both reaction products are complex substances.
For instance:
CH4 + Cl2 = CH3Cl + HCl