How do you think he stores his bitcoin? 18k BTC personal stash, and company coins 108k BTC ... ... You have that much, better put they keys in some building with armed guards. Be your own bank. Literally.
If we know the way they invest in BTC, and that is through Coinbase - then it is logical to conclude that they use Coinbase Custody as a way to store their digital assets. Personally, I don't know that Saylor ever posted anything about it, but I don't believe that he or anyone else from his company keeps any private keys, seeds or has direct access to the place where that BTC is kept.
In addition CC provides insurance for the funds it keeps, although I’m not sure up to what percentage.