What is the ruling on the insurance provided by companies for their employees, and the ruling on their evasion of it by writing different salaries, and is the employee a partner in sin if he signs a contract with a different salary to avoid losing his job?

Commercial insurance of all types is forbidden (haram). Therefore, it is not permissible for any company to insure its workers, unless it is compelled to do so, in which case there is no sin upon it, due to the saying of the Prophet (peace and blessings be upon him): "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are compelled to do." And it is permissible for it to evade it by any possible means.