All I could find is a declaration is actually one of their subsidiaries, called Microstrategy holding their bitcoins, as they previously disclosed.
It is not immediately clear why Microstrategy want to create a new company (that would presumedly be 100% owned by Microstrategy) to hold his bitcoins.
Digging on their website I also found a document with a checklist on the checklist used to determine which kind of custody of the coins:
Not much informations however on how this checklist should direct the decisions about this:
Lastly, a declaration of Micheal Saylor spoke vaguely about “self-custody” without going much into details.
MICHAEL SAYLOR: WE CUSTODY OUR BITCOIN AND DO NOT LEND IT OUT
In a recent interview at ETF Think Tank, Michael Saylor explained why his company purchases bitcoin directly, custody it themselves, and doesn't lend it out.
The fact that not many details about the custody scheme itself make me think. Security by obscurity is never a good approach, and this lack of details about the scheme makes me think it is way more weak than we think (thinking about the checklist mentioned in the first point).