What's the difference between using a laptop that has a hardware which have been certified for use with Ubuntu (Dell for example), and a laptop that has a hardware that haven't been certified for use with Ubuntu (Asus for example)?

Can a company use Ubuntu legally on laptops with hardware which haven’t been certified for use with Ubuntu ?

You seem to be asking two different questions.

  • Can a company use Ubuntu legally on laptops with hardware which haven’t been certified for use with Ubuntu ?

Generally ‘yes’. Refer to https://www.ubuntu.com/licensing.
And to https://askubuntu.com/questions/219/under-what-license-is-ubuntu-can-it-be-legally-modified-and-distributed

  • What does “Ubuntu Certified” mean?

Refer to https://certification.ubuntu.com/
And to https://askubuntu.com/questions/781123/does-ubuntu-supported-hardware-without-official-certification
…which is essentially a duplicate of https://askubuntu.com/questions/986878/will-my-device-work-with-ubuntu

Since these both seem to be support questions, rather than community or development questions, please follow up in one of the linked support threads.