One of the key parts of whatever definition you use is the emphasis on nation rather than nation-state (I know Britannica has an either-or in there, but generally, people use the term nationalism for the former). Germany is a nation-state. The German nation is a national identity that includes Germans both within and without Germany. German nationalism promotes the interests of the German nation, not Germany the state. I use Germany as an example, because it offers a readily accessible lesson in the excesses of nationalism. The German nation was never unified in the 19th C. largely because the Hohenzollerns didn't want to play second fiddle to the Habsburgs. But eventually Hitler came along, and German nationalism was an important component of Nazi ideology that directly led to much of his Eastern policy: the Anschluss and annexation of the Sudetenland to unify all Germans, and then Lebensraum to provide for the German people (at the expense of others, like the Slavs).
But not all instances of nationalism are so readily dismissed as dangerous or evil. Modern examples of nationalism that seem to be romanticized by many might include Scottish and Catalan nationalism, which are really just separatist movements, and don't appear to want to promote Scottish or Catalan interests at the expense of others. So I think one might argue that nationalism generically can be positive or negative, depending on how far one takes it, ranging from simple self-determination to supremacy over others.
However, in a multinational state like the US, most nationalist movements are not nearly so generic. You won't find a lot of American Nationalists (but see below re: MAGA). Instead, you have white nationalists and black nationalists, and so forth, and those modifiers don't just describe the members, but define the ideology, as well. So a White Christian Nationalist isn't just a nationalist who happens to be a white Christian. Rather, he's a nationalist that defines his national identity specifically as limited to white Christians, and will promote their interests over others. Taken to the furthest extreme, he will argue that non-whites and non-Christians aren't truly Americans and ultimately should be removed from the country.
These people truly exist, but I don't think they make up the majority of MAGA. I think MAGA are mostly nativists. They might be more comfortable around other white Christians, just for the familiarity, but they won't deny that a natural born black or Jewish citizen is or ought to be truly a member of the American nation. Rather, as is being discussed in another thread, the extreme ends of their ideology are found in their rabid opposition to immigration, both legal and illegal. In that sense, MAGA might be seen as a rather hardcore version of that generic American Nationalism that I said above is generally rare in a country like ours.