For example, how can I create a one-to-many relationship between tables
CREATE TABLE InsuranceCo (
id int PRIMARY KEY,
CREATE TABLE Vehicle (
licencePlate CHAR(10) PRIMARY KEY REFERENCES InsuranceCo(id),
This isn't how one-to-many relationships work at all. You don't just link two ids together, that is how one-to-one relations work (and no, those can't be done with different types, the values actually have to be the same). For one-to-many relationships, you need a separate value to reference the other table with.
You have to add a column -- for example insuranceCoId -- into the Vehicle table. Then any vehicle can have the id of the insurance company right there in the table. So data in might look like this:
InsuranceCo: id phone 1 800-744-2932 2 488-382-9332 Vehicle LicencePlate insuranceCoId year 435yte 1 1995 328teo 1 2006 fd8tew 2 2008
As you can see, one insurance company is associated with many vehicles now.