Do they use cold wallets for it like everyone else? If so, do they keep everything in one wallet or do they split the bitcoin into several different wallets?
Let's say 1/3 in Trezor, 1/3 in Ledger, 1/3 in BitBox02? I'm currently trying to transfer my bitcoin from an exchange to a cold wallet for the first time, but I'm wondering if it's safe to keep everything in one wallet from one company.
If you own thousands of BTC, you most probably want a dedicated, airgapped machine that uses nothing but Bitcoin Core or Electrum to generate addresses where you deposit your funds.
Whenever you want to spend them, you create the transaction, put it on a flash drive, bring that flash drive to a
different machine which does have internet access and broadcast it from there. You will also want your own node such that broadcasting large Bitcoin transactions aren't tied to your IP address and put you at personal / targeted risk of attacks (digital and physical).
My guess though, is that some of the large Bitcoin investors don't do this, instead they are probably so wealthy that they leave their BTC with an exchange or personal broker, in an insured fashion. Such that if the funds go missing or anything else, they can sue them to death. Just a hypothesis, though.