how backup iMac like can with iPhone or iPad in case something happens

See information in Move your content to a new Mac - Apple Support

Note that you can transfer from a Time Machine backup or directly from another Mac (if the old Mac is still working).

The following has information about how to backup to Time Machine (a good idea at any time even if not planning to move to a new Mac): Back up your Mac with Time Machine - Apple Support

Welcome to Apple Support Community.

To start a backup, you need an external hard drive. Here’s a few excellent ones sold by Apple: 

• LaCie Mobile Drive 4TB External Hard Drive USB-C - Apple
• LaCie 4TB Rugged USB-C Portable Hard Drive - Apple

If you have an iMac (Late 2015) or earlier, or a Mac mini (Late 2014) or earlier, you will also need this cable. 

You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. 

Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:

• Upgrade to iCloud+ - Apple Support
• Set up iCloud Drive - Apple Support
• Add your Desktop and Documents files to iCloud Drive - Apple Support
• Set up and use iCloud Photos - Apple Support

After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.

Jack

I am getting that English may be your second language. If I understand you, you want to transfer data from an older Mac to a new Mac. If I am correct, please follow the instructions in Setup Assistant and Migration Assistant.

However, if you are migrating from a PC, the easiest method is to move the data you want to migrate onto an external HD. Then connect the HD to the new Mac and drag and drop the data to the Mac. For example if you have a large photos library on your PC, first move it to the EHD. Next connect the EHD to the Mac, the drag the library to the Photos app icon in the Dock and all of your photos will be imported into the Mac's Photos's app. It is that simple.