As I explained last time, there are two basic categories of nationality words. One is where the adjective and the noun are the same: these nationalities mainly end in 'n' or 'i'. So you can say either 'Iranian' people' (using the adjective) or Iranians (using the noun).
You cannot refer to people from Iran in general as 'the Iranian'. Why? Because 'Iranian' is a noun: 'an Iranian' is one (non-specific) person, while 'the Iranian' is one (specific) person. For example, someone might say, "There were three candidates for the post: an American, a South African and an Iranian. The Iranian was the best candidate, so he got the job."
Now for the other category of nationalities: the ones where the adjective can't also function as a noun. With these nationalities, you can use the adjective plus a plural noun to talk about these people in general terms, the same as with the first category: 'French people' (without an article). You also have another option that only works with this second category: you can put 'the' before the adjective and refer to the entire French nation as 'the French'. [This is the same construction as when we use a 'the+adjective' combination to refer 'the poor' or 'the young' as an entire class of people]. Both are correct, but there is a subtle difference - 'French people' is thinking of them as individuals, whereas 'the French' suggests that they are one single group.
So what about 'the Iranian people' or 'the French people'? Well, here, the normal rules of using the definite article apply: we use this to refer to a specific group of people of that nationality. For example, "The Iranian people who live in my neighbourhood are mostly very friendly".
[You may occasionally see 'the xxx people' being used in the singular to refer to an entire nation. For example 'This is the will of the British people'. This is unusual, and tends to be confined to rhetoric. I wouldn't try to use this form, if I were you].
I hope that all makes sense.