The theory of multilingualism is based on the concept of an L1 and an L2 language. L1 is the dominant language, while L2 is the non-dominant. Both may be learned to fluency, however one is dominant and is utilised by the bilingual or multilingual individual as the foundation for the learning of all other languages. Even in a home where perhaps three languages are spoken fluently, and a child is exposed to all three equally, there will be a dominant language on which they have based the learning of the other two.
Let's equate it to learning math. Once a child learns simple arithmetic, they can use what they have internalised about the function of addition and subtaction to learning multiplication and division. Those functions can then be applied to learning algebra, and calculus, and trigonometry, and even chemistry, computer science, and cooking. What is the foundation for all of this applied knowledge? Simple arithemtic. An individual working a calculus problem does not go back to " It works this way because 2+2 is the basic principle of addition and to add means to increase." They have internalized that knowledge and it is applied without concscious thought. But at the same time, without knowledge of simple arithmetic and the priciple pf addition, one could not solve a calculus problem.
The same with languages. L1 is the basic arithemetic, and the principles learned and internalized through the dominant language is transferred to the learning of all additional languages. Once one understands that a word is a symbol for representing an object, then one can understand that all words in all languages are symbols for representing objects. The next step is learning that one can use those symbols to communicate basic needs and desires, and that if it can be done in one langauge, it can be done in all language. Then it leads to communication of emotion and of complicated thoughts and concepts. But it is all founded on the discovery with one symbol in one language. If a child first makes the discovery that the sign for "milk" is the symbol that represents the thing they want, then their L1 language is sign. If they discover that the spoken word "milk" is the symbol, then their L1 langauge is English. The discovery that both are symbols can be seconds after the initial discovery, but it doesn't matter that they happen almost simultaneously. One must always come first, and that is what constitutes the beginning of applying what one knows about the L1 language to the L2 language.
Conversely, if a deaf child is not in an environment that allows for this discovery, and must be taught through directed activity that langauge is system of symbols that represent the thing, then they become language delayed. A deaf child who is restricted to an oral only environment is impeded in taking in the peripheral information that allows for acquisition and internalization of the rules and functions of language. Their cognitive processes are restricted in the fluid ability to generalize what they know.