What's this constructed number's starter?
05AB1E, 7 5 4 bytes

&gt;9*н


Try it online!

&gt;            # input + 1
 9*          # * 9
   н         # take the first digit


The \$(n-1)^{th}\$ term of the sequence starting with \$a\$ is \$\sum _{i=1}^{n-1} a \times i \times 10^{i-1} = a \times \frac{(10^n - 1) / 9 - n}{9}\$. Multiply that by \$9\$ and add \$a\times n\$, and you get \$a \times \frac{10^n - 1}{9}\$, aka the digit \$a\$ repeated \$n\$ times. Turns out adding \$9\$ instead of \$a\times n\$ works for \$n=1\$, and for bigger \$n\$ the linear difference is negligible next to the exponential growth.