I would like create a custom page in the SPO using SPA with HTML,CSS and JS. I was not able to find a way to start with I want complete page or Full Page. I want create like webpage with responsive to all devices.
According to my research and testing, I recommend that you use SPFx to create web parts and put custom web parts on SharePoint Oline pages as needed to implement custom pages.
You can refer to this document to build the Web Part using SPFx: Build your first SharePoint client-side web part (Hello World part 1)
You have to use a regular SharePoint .aspx page with a Webpart, and then have your SPA loaded by this webpart. By using CSS, you can hide the extra SP stuff you don't want to show. I have an application that works this way. It's not optimal because all the SharePoint crap is loaded… but it's better than nothing.
In my case, I couldn't use SPFx with a "modern" page (because the app catalog was not available to me), so I used a Content Editor Webpart within a "classic" SharePoint webPart page. With this webpart you can load an .html page (built using NodeJS and webpack).