Actually I enabled Quick Text and it is working fine with Activities but i need that on Case Details Page field (type: Rich Text area) can we Do that on case detail page ? It is showing on Activities: But not on details page for Custom field on case can we do that or is there any thing else i can use like quick text?
No, you cannot.
There's an idea to have quick text used for standard case comments which you can vote for here: https://success.salesforce.com/ideaView?id=0873A0000003XnRQAU
Any other option would require some type of custom creation with LWC or something along those lines. It's potentially not worth the overhead considering the use case (one custom field on a standard layout).