I installed Emacs 26 on Ubuntu using snap.
Where to find the Emacs configuration file in this context ?
Once you have opened Emacs you can see what is your emacs directory by checking the value of the variable user-emacs-directory.
Just: C-hv user-emacs-directory.
Inside that directory an init.el should be the configuration file. If that doesn't work, you should check ~/.emacs or ~/.emacs.el files.
You can also find more details in the manual.