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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