How do insurance companies know about pre-existing conditions?

There are some insurance providers who, while determining an applicant’s pre-existing medical condition, will consider their medical history in entirety. Some other providers may only consider at the applicant’s medical history for a period dating back to the past 4 years.

https://www.bankbazaar.com/health-insurance/pre-existing-conditions-diseases-coverage.html#:~:text=There%20are%20some%20insurance%20providers,to%20the%20past%204%20years.