What differentiates fascism from just plain old militaristic imperialism?

This is a really difficult question to answer because neither fascism nor imperialism are eternal forms but historically and geographically contingent, for example 19th century European imperialism is very different from 20th century US and Soviet imperialism.

So I think in order to answer the question it makes sense to look at a concrete historical example. Let's take the imperialism of the German Kaiserreich vs. the fascism of the NS regime because I'm familiar with that. Even then, there are going to be necessary generalizations and simplifications so bear with me. So, first of all, there are some obvious similarities. Both were authoritarian forms of rule, both were nationalistic and both were politically and militaristically expansive. But there are stark differences in (1) how the regimes are internally structured, (2) how the regime legitimizes itself and (3) how the expansion is carried out and justified ideologically. In all three we can essentially see a break from 19th century aristocratic imperialism to 20th century totalitarian mass politics. As to (1), while the Old Germany was in a transition phase to 20th century politics during it's final decades, there were still large remnants of old, aristocratic, top-down politics. Now you might say, wait a minute, isn't fascism also a top-down system politically? But the way 20th century totalitarian ideologies functioned (Stalinism aswell) was precisely through a technocratic-bureaucratic diffusion of power, orders were given from the top but they were almost impossibly vague. Not only that but the system explicitely functioned in way were it wasn't totally clear which organ of the state was responible for carrying out the orders. In NS research these are called "double structures", which refers to two competing institutions which had overlapping areas of competency and were effectively competing with each other for political power. So the way fascism fundamentally functioned as a state in opposition to 19th century politics was in it's emancipation of it's institutions, the way it constructed the state not as as a stiff and metallic structure but as an organism (which, somewhat ironically, allows for a much larger control down to the micro-level than the "old system"). This theme of organic organization can also been in (2) quite literally, whereas the Kaiserreich still largely legitimized itself through the old form of aristocratic sovereignity, fascism legitimized itself through mass- and biopolitics as a quite literal kind of biological totality, the "body of the Volk", the Aryan bloodline. So accordingly, the Führer wasn't the political sovereign but merely the excutioner of the combined political will of this biological population, which makes up the real political body. This also affects (3), while there were racial motives in 19th century imperialism without a doubt, the imperialism of the Kaiserreich was still largely concerned with land as kind of abstract category, a way of gaining prestige and power in an aristocratic squabble with the other European sovereigns. As such, where the expansion is carried is almost secondary, what's more important is having more colonies than your neighbours. This can be seen in how the European imperial powers treated incredibly divergent political and social geography in the third world with a kind of blank slate, only concerned with amassing as much colonial land and maximizing the econimic production from those colonies. Fascism on the other hand, was concerned with an explicitely biological expansion. In this view, land and economic production are just means to the end of racial expansion. The way the NS regime treated different conquered areas based on the "racial distribution" of the population was incredibly acribic and differentiated. The point wasn't just to conquer land, any land, to further the power of the Nation but to "liberate" fellow Aryans and to subsume and ultimately exterminate the lesser races. This also how the expansion was legitimized politically, again, not as a political project of the sovereign or nation in search of abstract power but as the direct biological will of the combined body of the Volk.