I want to create the Modern SharePoint Page using any Custom Page Layout.
Is it possible using SPFx or server side code? If yes, Can you please share the code or any reference link?
You are defining alternative layout inside of the modern page.
Check this link there is written:
These customizations are currently not supported for "modern" pages:
- Adding "classic" web parts on "modern" pages Custom CSS via AlternateCSSUrl web property
- Custom master pages (more extensive branding will be supported later using alternative options)
- Minimal Download Strategy (MDS)
- Modern pages inside of the a Site Template package (Save site as a template)