how to cofigure imap

Gmail is set up as IMAP (a slightly weird version if I recall but not in a way that matters in this case) by default. When adding the Gmail account to your device, you should only have to select Gmail and then enter your email address and password. If the accounts are not syncing, I'd recommend you delete the account from all of your devices and add it back. That's generally the fastest way to resolve many email problems.

Add email accounts in Mail on Mac - Apple Support

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

I would note that I always access my Gmail account through a web browser on my Mac. I much prefer it to Apple's Mail application.

To setup iCloud as the Mail Account on all the devices

Suggest reading the article from Apple on this topic

Link below

Sign in to iCloud on all your devices - Apple Support (CA)

Apps and features that use iCloud - Apple Support (CA)

excerpt for above link

iCloud Mail

Create a free @icloud.com email address and send and receive mail on your devices and on the web at iCloud.com

How iCloud keeps information up to date across all your devices - Apple Support (CA)

Do you have the IMAP mailbox already set up on both devices and they are not syncing or do you have it set up on one device and need to know how to set it up on the other?

Who is your email provider?

If it doesn't work, post back and hopefully someone will have additional insight.

Best of luck.