Plain version: the application is the request; the policy is the contract itself, written down. And California tells insurers the minimum that written contract has to say.
California Insurance Code §380 defines the policy as the written instrument in which a contract of insurance is set forth. Compare: the application is the request for coverage, an endorsement changes the policy, and the declarations page is one part of the policy rather than the whole contract.
Section 381 lists six things every policy must specify:
The parties between whom the contract is made
The property or life insured
The insured's interest in the property, if he or she is not the absolute owner
The risks insured against
The premium; if the exact premium can be determined only when the contract ends, the policy states the basis and rates for calculating it
Memory trick: four P's (parties, property or life, period, premium) plus interest and risks. The agent's commission is not on the list.