Where is mobilesync folder on iMac?

They should all be there as far as I recall, but only one backup per device, except maybe in the old days...

Backups from your computer

A computer backup of your device, which is not the same as a sync, includes almost all of your device's data and settings. A backup from a computer doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to Apple Books
• Content synced from Finder or iTunes, like imported MP3s or CDs, videos, books, and photos
• Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages
• Face ID or Touch ID settings
• Apple Pay information and settings
• Apple Mail data
• Activity, Health, and Keychain data (to back up this content, you'll need to use Encrypted Backup in iTunes.)

Learn how to make a backups from your computer, how to find them on your Mac or PC, and how to delete backups that you no longer need.

Can I use my device's backup for another kind of device, like an iPhone backup for an iPad?

You can restore a device from a backup of another kind of device, like using an iPad backup for an iPhone, but certain types of content won't transfer. This content includes photos, Messages and attachments in Messages, Voice Memos, and apps that aren't compatible with the device that you’re setting up. For example, apps compatible only with iPad won't transfer to your iPhone.

https://discussions.apple.com/thread/7331041?answerId=29298926022#29298926022

https://www.intego.com/mac-security-blog/the-ultimate-guide-to-backups-and-storage-for-iphone-and-ipad/#:~:text=Storing%20multiple%20backups,space%20on%20your%20hard%20drive.

Backups on your Mac

To find a list of your backups:

1. Click the magnifier icon  in the menu bar.
2. Type or copy and paste this: ~/Library/Application Support/MobileSync/Backup/
3. Press Return.

To find a specific backup:

1. On a Mac with macOS Catalina 10.15 or later, open the Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes. Click your device.
2. In the Finder, under the General tab, click Manage Backups to see a list of your backups. From here, you can right-click on the backup that you want, then choose Show in Finder, or you can select Delete or Archive. In iTunes, choose Preferences, then click Devices. From here, you can right-click on the backup that you want, then choose Show in Finder, or you can select Delete or Archive.