Different parts of the constitution were written at different times. Amendments are part of the constitution regardless of when they were written.

Originalists interpret the applicable parts of the constitution as they were written, when they were written.

Search Scalia on the topic and watch some videos. He goes into pretty interesting depth on the subject. It's pretty fascinating.

https://www.c-span.org/video/?292678-1/justices-breyer-scali...