How would I know what data are stored in iMac and iCloud?

Nykaylla wrote:

I have been backing up to an external hard drive but haven't lately

You can log into iCloud from another computer to see what files are there in your account.

You said your Mac will not start up. Does it power on at all? If not, then it may have hardware damage or possibly it is not receiving power from the wall outlet, try testing the outlet or using a different one in that case.

If it powers on but shows a folder with a question mark, or otherwise does not complete the start up process, try booting into Recovery to run Disk Utility and repair the start up drive:

Use macOS Recovery on an Intel-based Mac - Apple Support

Use macOS Recovery on a Mac with Apple silicon - Apple Support

Nykaylla wrote:

We had a power outage and now my mac will

Not start. I thought all

My files are stored on the cloud. I am

Confused what is actually stored on my mac and what isn't. Someone please explain this to me?

Your files are stored on your Mac unless you have made some special arrangement to store them in the cloud.

That also implies that it is your responsibility to back up your Mac to protect against losing your data if some bad thing (fire, theft, lightning strike, power outage, hardware failure) should happen to your locally-stored data.