From an earlier answer of mine

This is really two questions, as the other commentators have pointed out the Germany of the Third Reich and the Kaiserreich were different governments fighting different wars that had different objectives. There was some overlap between the overall geostrategic context and political context that pushed Bulgaria to Germany's side, but even here there were major differences between 1915 and 1941.

Bulgaria's entry on the side of the Central Powers was a mixture of opportunism and fear on the part of the Sofia. Tsar Ferdinand and his ministers had briefly considered joining the Entente in the early stages of the war and this would have been a natural evolution of Bulgarian foreign policy. Bulgaria had already fought the Ottomans in the Balkan Wars and most Europeans considered Bulgaria to be the most dogmatically pro-Russian states in Europe. St Petersburg's lack of support to Bulgaria had alienated Ferdinand and his chief minister from Russia and Sofia had started to gravitate towards Germany before 1914, but the situation was still fluid during the war. The stalling of the Gallipoli campaign and the joint Austro-German invasion of Serbia showed Sofia that the Central Powers had a degree of ascendancy in the Balkans. Sofia had floated to the Entente belligerency for the price of significant territory from the Ottomans, but the Allies were unwilling to match this price. Berlin had no such compunctions and persuaded the Ottomans to cede the Maritsa valley and promised Sofia large concessions in Thrace.

Bulgaria's actions in 1915 appear very much as jackal-like opportunism, but Sofia was in a difficult position. The German occupation and exploitation of Belgium the prior year had underscored the dangers of being a neutral state that was geographically athwart Germany's strategic interests. Bulgaria did not have the strength to stand against the Central Powers without the direct aid of the one of the larger Entente powers; the much larger Romania learned this lesson in 1916/17. Lacking any direct geographic connection with Entente, an alliance with the Central Powers was the best of a set of bad options for Bulgaria.

For its part, Berlin welcomed having an ally in the Balkans and Bulgarian belligerency meant the Ottoman's northern flank was secured. Yet the alliance had yet to meet the acid test of actually having Germany own up to its promises of territory to Bulgaria. Most German diplomats did not look forward to the postwar negotiations of an enlarged Bulgaria, especially as it would have led to inter-Central Power squabbles over the Balkans. The German military recognized the immediate utility of Bulgarian belligerency, but did not place much hope in Bulgarian arms. This carried over into the postwar period where both Ludendorff and von Hindenburg scapegoated Bulgaria's collapse for the timing of the German armistice. As with many alliances that prioritized immediate needs, there was little thought into the long-term ramifications of these policies.

The Third Reich's approach to Bulgaria was a little bit different than its imperial forebearer. Although National Socialist thought towards the Balkan peoples did consign them to racial inferiors, such ideology was far from complete. When it came to conceptualizing a hierarchy of racial inferiors, Nazi thought was preoccupied with Jews and the Slavs of the USSR and Poland. The Balkans were peripheral to this thinking and more often immediate, practical concerns trumped racial ones. German economic domination of the Balkans and the construction of a RM-bloc in the region meant that it had to deal with the existing Balkan governments. Arms sales to the Balkans provided the Reich an infusion of hard currency, minerals, and raw materials, thus injecting a small measure of stability into the precarious German economy.

This sort of economic imperialism and soft power had much deeper roots than Hitler. Even as early as the nineteenth century, German industrialists and diplomats looked to the newly-independent Balkans as a potential economic vassal or dependency of Germany. This line of thinking continued into the Weimar period as economic partnerships would undercut the France's "Little Entente" of the Eastern Europe. But Hitler's government injected a more dynamic and exploitative round of this relationship.

As in 1915, Bulgaria was in an unenvious position. The Depression had underscored the need for foreign trade and the Germans were able to set terms. The communist USSR was not a politically-viable ally for the increasingly autocratic and conservative Bulgarian state. The was only increased Bulgaria's isolation as the Molotov-Ribbentrop Pact gave Stalin a free hand in rearranging Eastern Europe's borders and the defeat of France in 1940 made Germany the only viable ally on the continent.

Yet Bulgaria was the least trusted of the minor Axis states. As in the prior war, the Germans offered territory for Bulgarian belligerency and secured transit rights for Axis forces through the country. Bulgaria though was only to play a subsidiary and supporting role in Barbarossa. The main goal of Bulgarian forces was to ensure the Turks did not enter into the war against the Axis and hold Greek and Yugoslavian territory. Racialist thought certainly played a role in this subordinate role; when the German ambassador asked Berlin why Bulgarian troops were not requested for the invasion, Berlin responded that there was a danger of "fraternization between Bolshevik and Slav" that should be avoided. OKW only requested Bulgarian troops in the USSR after 1942 when manpower shortfalls among the Axis coalition became acute.

Sofia's response to being a reluctant Axis member though was far from complimentary to Tsar Boris III's government. Sofia sought at all costs to maintain a degree of independence and latitude from Berlin. Bulgaria, for example, maintained diplomatic relations with Moscow in spite of Berlin's requests to break them. Yet the German tether had hard limits. The government did bow to German pressure to introduce antisemitic legislation against its Jewish population. Sofia did resist German pressures to deport its Jews, partly because of pressures from Bulgarian civil society as well as the Orthodox church. But Sofia's foot-dragging on Jews was all too typical for the post-Stalingrad Axis powers. Giving into German requests opened up the danger of ever-increasing German demands on the Axis powers in a period when it was clear Germany was going to lose the war. So refusal on the Jewish deportations often had tactical political elements, showing to both Germany and the Allies that the Axis states were not merely Hitler's vassals. Jewish populations that did not suit the purposes of reasons of state were much more vulnerable to German pressures. Bulgaria here was no exception and the Jews in areas occupied by the Bulgarian Army were deported with Bulgarian assistance and very few of them survived.

One of the common traps in Third Reich studies is to assume that the Nazis maintained a coherent ideological vision that schematically divided the world. While such divisions of the world may have been found on the pages of Völkischer Beobachter, the world was far too complex to fit in this mold. Whether it was accepting the need of Balkan Slavs to aid in its anti-communist crusade or enlisting the Japanese to tie down American and British power in East Asia, Hitler found that even die-hard racists could not afford to be too picky when it came to fighting a modern global war. The immediate needs of Germany often outweighed Manichean racialist precepts even though Nazi racism often made these relationship unworkable over the long-term.

Sources

Gross, Stephen G. Export Empire: German Soft Power in Southeastern Europe, 1890-1945. Cambridge: Cambridge Univ Press, 2017.

Hall, Richard C. Bulgaria's road to the First World War. Vol. 460. East European Monographs, 1996.

Militärgeschichtliches Forschungsamt. Germany and the Second World War. Volume IV: The Attack on the Soviet Union. Oxford: Clarendon Press, 1998.