Hi,
Default behavior is that when writing an email, each time you press enter it creates an empty line.
1°The option is not so easy to find and that it why you see so many online posts providing a solution to this. This might annoy new thunderbird users, especially old people who just want to write as they usually do anywhere else.
Libre office, gmail, outlook, don't default their editor adding automatically a new line while pressing enter so why Thunderbird?