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.