does << (input redirection in append mode ) works only for newline?

while reading about file creation in linux I got :

Another way to create a file at the terminal is cat > <filename> << EOF. A new file is created and you can type the required input. To exit, enter EOF at the beginning of a line.

so I tried this on my shell prompt

$ cat > test1.txt <<ABC
> This is just a test file.
> ABC
$ cat test1.txt
This is just a test file.

here when I typed ABC(it could be anything - just for matching pattern) .in the newline it worked(accepted as it would have done in case of ctrl + C),but when I tried

$ cat >> test1.txt <<ABC
> just for a trial
> though ABC
> it didnt worked.
> ABC
$ cat test1.txt
This is just a test file.
just for a trial
though ABC
it didnt worked.
$ 

doing this , it only worked in the newline. Does it mean that << works for newline,

And what's going on inside cat> test1.txt <<ABC exactly ??