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.