Append file

To append the current file to a diffierent file, use the write ex command.
:w >> filename
The file you append to will become the alternate file.
Alternatively you can append a range instead of the entire file. By default :w takes :1,$ as the range. You could append lines 5 through 42 to filename.txt with
:5,42w >> filename.txt

Further Reading

:h :w_a

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