You'll need an external optical drive to do that.

Apple's own application will play commercial DVDs, provided you supply a compatible optical drive. The DVD Player app is installed as part of macOS and is located in your Mac at

Typically the application will launch when you insert a DVD into a connected and working optical drive. DVD Player User Guide for Mac - Apple Support

Apple no longer sell their own SuperDrive optical drive, but you can find may relatively inexpensive drive is you search Amazon or google it with your browser. Many here will recommend the optical drives from OWC/macsales.com.

Some drives may come with their own AC power adapter (best), while others will pull their power directly from the USB connection on the Mac. Some drives don't like to be connected to the computer through a dock or hub, working best or only when they are connected directly to the computer.

Your iMac has USB-C ports, so the optical drive you buy should preferably come with a USB-C cable or connector. However, most optical drives with even a USB-A plug can be adapted with a to work just fine.

I've had good luck with Asus and OWC brands of optical drives as well as the now discontinued Apple SuperDrive on many Macs olde and new.

As for ripping your owned DVDs to be stored on your hard drive, HandBrake works well.

D.I. Johnson wrote:

Your iMac has USB-C ports, so the optical drive you buy should preferably come with a USB-C cable or connector. However, most optical drives with even a USB-A plug can be adapted with a to work just fine.

In the case of the desktop CD/DVD and CD/DVD/Blu-Ray drives from OWC, those have USB 3.0 B sockets and detachable cables that go from USB 3.0 B to USB-A.

You can easily get replacement cables from other places, including USB 3.0 B to USB-C cables that cut out the need for a separate USB-A to USB-C adapter.

Just to add, you will need third-party software to play Blu-ray DVDs.