ci-3 how to create admin without duplicating core files

thanks for the reply [ @InsiteFX @Ivo Miranda @skunkbad Wrote] ... i mean if i enter www.example.com/admin, it should go to admin login page,so how will be the directory structure? which means separate front-end and back-end.

ex:
if its user side :
/application

admin:
/application/admin [here all the admin files].