Answer by Kamil Maciorowski for How are ctags and etags actually defined in Linux (Ubuntu)?

man ctags states:

default is tags, or TAGS when running in etags mode

And about the etags mode:

[…] will create a tag file for use with the Emacs editor. […] if ctags is invoked by a name containing the string "etags" (either by renaming, or creating a link to, the executable), etags mode will be enabled.

For how ctags-exuberant knows the name it is invoked by, see this answer.