How to work with Security in Symfony?

Question

symfony

I had to deal with a legacy project that was half rewritten in Symfony 3. Symfony itself is not a big connoisseur.
It was the turn of the admin panel, the question arose of how to organize the separation of rights and authorization. The difficulty is that authorization now works through the old code, and some controllers have already been rewritten for symphony. What code to use is decided by a small helper, but how to make it possible to authorize both in the old and new places of the user, and even check the differentiation of rights?
Now authorization happens in legacy , can I authorize a user there and in Symfony at the same time?
Is it worth cutting your own solution or will some FOSUserBundle, for example, fit?
I will be grateful in advance for sensible examples of the implementation of authorization and the separation of rights.