I upvoted @Chanmunka's answer, but let me add an additional thought that's too long to fit in a comment:
Yes, the usage is inconsistent, or at least appears inconsistent.
For some nationalities, the noun for a person from that place and the adjective for that place are the same. "Bob is an American. Bob drives an American car." "Erwin is a German. Erwin drives a German car."
For others the words are different. "Richard is a Briton." But, "Richard drives a British car."
Oh, and by the way, often you make the adjective by dropping any final vowel and adding -an to the noun. Like America -> American, Mexico -> Mexican, Venezuela -> Venezuelan, Russia -> Russian. If it ends in -y you change the -y to an -i-: Italy -> Italian. But for some reason Canada does not become "Canadan" but Canadian. Other times we drop the final syllable and add -ish. Like Britain -> British, Poland -> Polish, Ireland -> Irish. And then there are always the weird cases, like Netherlands becomes not "Netherlandan" or "Netherish" but Dutch.
For many nations the convention used to be that we used the adjective plus -man or the name of the place plus -man to refer to a person from that country. "Francois is a Frenchman." "Li is a Chinaman." Etc. As the conversation on another post indicated, some now consider that sexist, but no clear alternative has become accepted, so we're rather stuck using a phrase, "Francois is from France" or "Francois is a French person."
When the adjective is different from the noun, you can use "the" plus the adjective to describe the people of that nation as a whole. "The British like to drink tea." "The Chinese built the Great Wall." When the adjective is the same as the noun, you use the plural to describe that nation as a whole. "Germans are hard-working people." "Italians are crazy." Etc.