You're right, page viewer web part is not supported in SharePoint Online modern experience.

As a workaround you can use below web parts:

1. To show the file, use File viewer web part
2. To show the site pages, use Embed web part

You're right, page viewer web part is not supported in SharePoint Online modern experience.

As a workaround you can use below web parts:

1. To show the file, use File viewer web part
2. To show the site pages, use Embed web part

Update from comments:

If you want to develop your own web part, you can use the SharePoint Framework. Check below documentations for more information:

1. SharePoint Framework overview
2. SharePoint Framework web parts
3. SPFx web part samples