MicroStrategy is funding those buys with convertible debt so the ones buying those bond they can choose if they will get paid in stocks or cash when the bonds are mature so if the stock of MicroStrategy is low they will demand to get paid in cash MicroStrategy Revenue is 500 million annually and their debt 2 months ago when i made the research was 4.7 Billion dollars.

Debt to Equity Ratio was 111.6%
Market Cap to Revenue 210X

So MSTR basically an overleveraged bitcoin ETF because they bought their BTC with debt .

You can read more here