how do you lot view archived emails in gmail?

If something genuinely isn't in All Mail, check Spam and Trash next - archived mail is never deleted, but stuff that went straight to spam often gets remembered as archived. Also look under Settings, Filters and blocked addresses. A filter set to skip the inbox archives mail the moment it arrives, so you never saw it in the first place.

On the desktop it's hiding behind More at the bottom of the left sidebar - that labels list is collapsed by default so All Mail sits below the fold and nobody finds it. Archived mail lives in there, mixed in with sent and everything else. If you want a message back where it was, open it and use Move to Inbox rather than dragging it around.