How does bagging help reduce the variance

High Variance - Model varies a lot on small changes
High Bias - Model doesn't vary so much but predict quite away from the truth

Let's check a Decision Tree on 5 values - \begin{array} {|r|r|} \hline 1 &5 &10 &15 &20\\ \hline \end{array} In this tree split,
Value of 9.9 will be 7.5
Value of 10.1 will be 12.5.
Showing a very high variance.

Let's create 4 Random Tree of 3 elements each - \begin{array} {|r|r|} \hline Tree-1 &5 &10 &15\\ \hline Tree-2 &1 &15 &20\\ \hline Tree-3 &1 &05 &20\\ \hline Tree-4 &5 &15 &20\\ \hline \end{array} Value of 9.9 = (7.5 + 7.5 + 12.5 + 10)/4 ~ 9.375
Value of 10.1 = (12.5 + 7.5 + 12.5 + 10)/4 ~ 10.625
Variance is reduced a lot.

In bagging, we build multi-hundreds of the Tree(Can build other models too which offers high variance) which results in a large variance reduction