Where to add/edit meta tags data (description, keywords) in admin panel?

Upon creating new page/post, how can I add/modify meta tags content from the admin/editor panel?

You can do it globally (see here https://learn.getgrav.org/admin-panel/configuration-site#metadata ) or for every page (read the docs here https://learn.getgrav.org/admin-panel/page-options#publishing ).