Subsituting contents of entire file to middle of another file using awk

Hi

I have a data file 'File2' consisting of 105670 lines. I want to copy and paste 17928 lines from 'File1' to 'File2' but I want to place it in between lines 21 and 17950 of 'File2'. How do I do it in awk?

For example-
File A has 5 lines
X
Y
1 2 3
4 5 6
7 8 9

and File B has
A
b
C
124234
214353245
234234

Using AWK, I want to paste the last 3 lines of File A in between lines containing 'C' and containing '124234' of File B. For my actual files, I know the line numbers between which I want to paste.
I have used the 'NR' option in awk using line numbers for data processing. But I am confused about this code?

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