Sent mail on Windows 10 does not show up on iPhone. Sent mail on iPhone does not show up in Windows?

Sent mail on Windows 10 does not show up on iPhone. Sent mail on iPhone does not show up in Windows?

That depends on what email service you're using, and how.
If both devices send email via SMTP (the usual protocol to SEND email), then copies of sent mails are only kept on the sending device (if the sending device not was configured to delete mails that have been sent successfully).

If, however, you use a mail service that saves both sent and received emails and use IMAP (rather than POP3) to sync the mailbox contents, then you might get copies of your sent emails in a separate folder.

If you want a less generic answer, please add information (as a clarification) about what email service you're trying to sync against.