Theory:

The greatest one-digit number is \(9\). Adding \(1\) to the greatest one-digit number will result in  \(9+1 =10\). That is, it results in the smallest two-digit number.
 
The greatest two-digit number is \(99\). Adding \(1\) to the greatest two-digit number will result in \(99+1 =100\). That is, it results in the smallest three-digit number.
 
Proceeding in this way, we can have the following table.
 
Greatest numberAdding 1Smallest number
\(9\)\(+1\)\(=10\)
\(99\)\(+1\)
\(=100\)
\(999\)\(+1\)\(=1000\)
\(9999\)\(+1\)\(=10000\)
\(99999\)\(+1\)\(=100000\)
\(9999999\)\(+1\)\(=1000000\)
\(9999999\)\(+1\)\(=10000000\)
 
Thus, the resultant pattern becomes,
Greatest single\((1)\) digit number \(+ 1 =\) smallest \(2\)-digit number
Greatest \(2\)-digit number \(+ 1 =\) smallest \(3\)-digit number
Greatest \(3\)-digit number \(+ 1 =\) smallest \(4\)-digit number
and so on.
Important!
Recall the following:
\(1\) tens \(= 10\) ones
\(10\) tens \(= 1\) hundreds \(= 100\) ones
\(1\) thousand \(= 10\) hundreds \(= 100\) tens
\(1\) lakh \(= 100\) thousands \(= 1000\) hundreds
\(1\) crore \(= 100\) lakhs \(= 10,000\) thousands