Hello portenita,
Having messages sync across all your Apple devices is a great feature offered by Apple. You will need to be signed in on all your Apple devices using the same Apple ID and password. You will need to enable Messages in iCloud across all your Apple devices.
You can learn how to set up Messages in iCloud for all your devices here: Set up iCloud for Messages on all your devices - Apple Support. It offers steps for iPhones, iPads, and Mac devices.
Note: Make sure you’re signed in with the same Apple ID on each device. If you have devices where you’re not signed in with your Apple ID or that have Messages for iCloud turned off, your messages won’t be kept up to date on those devices.
We hope this information helps you out.
Cheers!