There are several ways to combine the contents of files. One of the easiest ways is to use cat command.
Suppose I have 3 files a1.txt, a2.txt, and a3.txt with the following contents. I can use cat to combine them.
$ cat a1.txt
$ cat a2.txt
$ cat a3.txt
$ cat a1.txt a2.txt a3.txt >a.txt
$ cat a.txt
You can choose the sequence of files in cat command depending on how you want to append the data.