Please explain the steps to balance a reaction using OXIDATION NUMBER METHOD

Q.

Explain with example how oxidation number helps us to balance a chemical reaction ?

Q. Balance the equation using oxidation method in steps
Zn+ HNO3 ------- >ZN(NO3)2 +NH4NO3+ H2O

Q. Hg2Cl2 (s) + SnCl4 (aq) → 2HgCl2 (aq) +SnCl2(aq)
​​. Removal of
electropositive element

how is this a oxidation reaction ? And which element is oxidised ? Please explain this reaction step by step .

Q. $C{l}_2\left(g\right)+O{H}^{-}\left(aq\right)\to OC{l}^{-}\left(aq\right)+C{l}^{-}\left(aq\right)$
Balance the above reaction using oxidation number method.