Can you use 1 large SSD device for booting and data (2 partitions) on Pi4?

You wrote:

I want to use one partition for booting (RASPIOS) and one for the data files.

The Raspberry Pi OS uses two partitions by default: one for booting, formatted as fat32, and one as root filesystem, formatted as ext4, containing all modules and programs to run the operating system. With a default Raspberry Pi OS the second partition is expanded to its maximum size on first bootup of the image. If using a big SSD device there is a lot of free space available on the second partition you want to store data files. Just create a directory:

rpi ~$ sudo mkdir /data

and use this directory as storage for the data files.

Yes, you can partition an SSD.

If you're using Windows you can use this guide to partition your SSD: https://www.wikihow.com/Partition-a-Drive-on-Windows

You can then use a tool like Etcher to flash the .iso to the partition you created for your OS.

You can use the other partition for normal storage.