What is the simplified legal book that a non-specialized person can suffice with to learn the correct creed according to the methodology of the Salaf, and are there explanations for it available online?

No single book by a knowledgeable scholar can be exhaustive in every respect. Many beneficial books are well-known. The important thing is to begin reading and acquiring knowledge, because preoccupation with asking about books, methods of acquisition, and prioritizing among them distracts from commencing studies. There are previous fatwas (121056, 63874) that clarified useful books for beginners in creed and other subjects. Many of these have audio شرح (explanations/commentaries) available online. So choose what you wish from them and start with it.