Strategy (MSTR) pays interest on its debt via a mixture of capital markets, software business cashflow, existing reserves & investments and Bitcoin sales.

MSTR's annual coupon payments are not that high - ≈$40M/year. The convertible bonds that they issue come with small coupons (e.g. 0%, 0.625%, %0.875, 2.25%)

Given MSTR's software business revenue of ≈$500M/year, but it actually operates at a ≈$60M loss after expenses are accounted for.

So effectively, strategy pays interest expenses from the same corporate funds bucket that they rase to purchase Bitcoin in capital markets.