I imagine this has been answered. But if I have a unit (mortar) at base cost 1945. But I pick the conscripts civic of 30% reduction cost, I have a barracks, then I choose ottomans which give heavy machine weapons 50% discount, Aztec 20٪ discount, germans 20% discount. How does the math work?

I haven't tested this for units, but one thing I know from the Inca (25% territory attach cost reduction) is that it seems to be additive - so 4 EQs make the cost 0. Similarly, the Argentinian EQ gives 4% military cost reduction, and that also seems to go to 0 rapidly (easy to have 25 territories on a large map). These would not be the cases if they were multiplicative as the previous commenter wrote. Paradox games also do this so I'm familiar with it - for reductions it works to your favor, for additions, it's not.

If someone else can verify, its an interesting topic since I'm doing a military run and there are several unit cost reduction traits and tenets.

Same effect adds up, different effects multiply. Thats why the Inca and Argentinian effects add up : they're from the same source (beef and eqs in the city). The multliple unit cost reduction effects come from different sources (LTs from different cultures, a civic, different infrastructures, etctera) so they multiply