problem with Advanced Custom Fields

I have a website which I have not created my self. After a lot of search I understood that my custom fields for the posts are made with Advanced Custom Fields.
But the strange part is that in my plugin I don’t have Advanced Custom Fields. Is it possible?
Right now I want to add some more custom fields. Is it OK if I install the plugin?

P.S. I have the class “acf-field acf-field-text acf-field-5cc9bdf14b927” for div tag of custom fields and in this way I understood that this plugin is used in my site