The number $9$ has the property that $10^n \equiv 1 \pmod 9$ for all $n$, so a given digit makes the same contribution to the value of a number modulo $9$ regardless of its position. It's not hard to see that the only numbers for which this is true are $1$, $3$, and $9$. In our case, reducing modulo $1$ gives no information but reducing modulo $3$ does not lead to a solution of the above form, as the number is congruent to $0$ modulo $3$ and there are squares with this property. In summary, only the choice $9$ works here.
NB we can frame your solution as follows: Reducing gives that the $10$-digit number is congruent to $3$ modulo $9$, so the number is divisible by $3$ and not by $9$ and hence is not a square (as its prime factorization contains a single factor of $3$).
Edit OP has removed some of the content of the question since this solution was written.