Can i use my existing Apple ID(Ipad) one to my newly purchased mobile?

Yes. You can and should use the same AppleID for all your devices - as doing so allows you to access all Apps and Services for which you have paid or subscribed.

If you choose to use a different AppleID, your iCloud data will not be accessible to all devices: your core Apple Apps (such as Calendar, Contacts, Notes, Reminders and Photos) will also not synchronise between your devices.

The iPad User Guide for iPadOS is a rich source of information from which to learn about your iPad and the ways that your devices share and synchronise their data and services. Here are links to both the downloadable and online versions of the Guide:

https://help.apple.com/epub/ipad/en/

https://support.apple.com/guide/ipad/welcome/ipados

yes you are suppose to use the same appleID on all apple products, it's sort of your social security number in all things apple