How did the National Socialists deal with the churches in the Weimar Era? And what were their (public) policies and beliefs?
They demanded freedom of religion, provided they do not threaten the German Race. The party stood for Positive Christianity but was not bound to any creed - as point 24 of the party programme dictated, something crafted mainly by Alfred Rosenberg and Dietrich Eckart, later called the "spiritual father" of the Third Reich, and influenced by the whole Germanist scene of the early 20th century, especially the Germanen Order and the Thulean Society that filled the ranks of the early party.
Now that was a purposefully broad descriptor. But why would they call the christianity of the main churches as "negative"? Here is some on their beliefs and works:
"In the case of the struggling young Christianity, for example, the Jews, quick as a flash, began hanging onto its coattails. Consider Paul, properly called Schaul, who was a rabbinical student. As a Jew, Paul certainly knew that of all the peoples of the world the Jews, first and foremost, needed their souls saved. 'Go not … to the Gentiles, … But go rather to the lost sheep of the house of Israel,' demanded Christ (Matthew 10:56). Paul ignored it. He went to the Greeks and the Romans and brought them his 'Christianity.' A 'Christianity' with which the Roman Empire became unhinged. 'All men are equal! Brotherhood! Pacifism! No more privileges!'… And the Jew triumphed." - Dietrich Eckart, "Bolshevism from Moses to Lenin, a conversation between Hitler and Me" (Eckart was honored in the second book of Mein Kampf)
"Today we recognise that the highest central values of the Roman and protestant churches, as a negative form of Christianity, do not correspond to our soul, that they stand in the way of the organic powers of the peoples determined by the Aryan race, that they have to make way for the latter, must allow themselves to be revalued in the sense of a Germanic Christianity. This is the meaning of the present search after religious truth." - Alfred Rosenberg. "The Myth of the Twentieth Century".
"The Party stands on the basis of Positive Christianity. Christianity is not dependent upon the Apostle's Creed" - Hans Kerrl, Reich Minister of Church Affairs.
Thus, under such arrangement they could use christian symbology and history, without being bound to christian doctrines or politics, something that was tolerable to the many vocally anti-clerical NSDAP members (like Rosenberg, Himmler, Hess, Bormann, Darré, Goebbels, Ley, Schirach and so on). It was also an important thing in order to gain power in a very christian country, and especially the loyalty of elements of the army, which always had shaky loyalty to the NSDAP and required massive bribes not to turn entirely inoperational because of treason, like Italy (https://en.wikipedia.org/wiki/Bribery_of_senior_Wehrmacht_officers).
It's important to notice, though that these efforts failed, due to the incessant opposition of cucked priests and bishops. Political christianity, the party of Angela Merkel today, was, at times, the main opponent of the NSDAP inside Germany - the priests opposed eugenics because "abortions are sinful" and even propped up militant groups like "The White Rose", in the middle of the war, effectively siding with the Rothschilds and the Soviets over the European people. No longer in need to gain power, the whole Positive Christianity psy-op and point 24 of the party programme were abandoned by 1941, leading to things like Bormann stating "National Socialism and Christianity are irreconcilable" publicly and Hitler ordering Rosenberg to draft a 30-point plan of a National Reich Church to replace both denominations.