If you start your sentence with "Most of the people", which is a plural noun phrase, naturally it has to be followed by a plural noun "smartphones":
Most of the people have smartphones nowadays.
This sentence does not indicate that each person possesses two or more smartphones. It's just saying that the number of smartphones each person has adds up to a number more than one.
How many smartphones each individual has is not indicated in that sentence. If you would like to say that most of the people have smartphones and that each person may either have one smartphone or more than one, you could say:
Everyone has at least one smartphone nowadays.
Here, as you already know, "everyone" is singular i.e. it refers to each single person instead of a whole bunch of them, so you can be more specific on how many / at least how many / up to how many smartphones each person has.
And yes, "the" can be used when you're referring to a specific type of smartphone:
- Most of the people use the iPhone 12 nowadays.
- Everyone uses the iPhone 12 nowadays.