California Realtor here. Escrow is opened by the listing agent and 99% of the time the seller/listing agent chooses the escrow company. Escrow is opened once there's a contract.
How do you open an escrow account? For home buyers and sellers, a real estate agent will typically open an escrow account on your behalf. However, if you need to open one, you simply need to contact a bank and ask to open an escrow account.