If so, do they keep everything in one wallet or do they split the bitcoin into several different wallets?
They would store their Bitcoin in different wallets and with different wallet types.
It is risky if you use only one wallet brand to store your Bitcoin. Always blackswan event can happen.
It is less risky if you store your Bitcoin in different wallets (with different keys/seeds) with one wallet brand.
It is more less risky if you store your Bitcoin in different wallets with different wallet brand.
So if one brand has issue, you still have Bitcoin in wallets from other brands.