Merge numbers to a new number and find digit on position \(n\)

The number you are constructing is related to the base 10 Champernowne constant, except that the Champernowne is a real number that starts with 0.123... while your "number" is an integer with an infinite number of digits.

In Mathematica, the Champernowne constant in base 10 is given by ChampernowneNumber[]. To get the 10,000 digit, just use RealDigits:

RealDigits[ChampernowneNumber[], 10, 1, -10000][[1, 1]]

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