Hi jjaramillo1005,
If your Mac is compatible, you may be able to install Big Sur on a new volume of your built-in startup disk and your Mac can start up from either volume.
"Add an APFS volume
- Make sure that your Mac is using macOS High Sierra or later.
- Open Disk Utility, which is in the Utilities folder of your Applications folder.
- In the sidebar, select your existing APFS volume, such as Macintosh HD.
- Choose Edit > Add APFS Volume from the menu bar, or click the add volume button (+) in the Disk Utility toolbar. These options are available only when an APFS-formatted volume is selected.
- Type any name for the new volume, then click Add.
- Quit Disk Utility.

Learn more about adding, deleting, or erasing APFS volumes.
Install a compatible macOS on the new volume
If the macOS that you want to install is compatible with your Mac, you can install it on the new volume.
- To install the latest beta (prerelease) version of macOS, enroll your Mac in the Apple Beta Software Program and follow the installation instructions on the program website.
- To install the latest release version of macOS, download it from the App Store.
- To install a version earlier than the version currently installed, use a bootable installer of macOS High Sierra or later.
When the installer opens and you're asked to choose where to install, click Show All Disks, then select the new volume before continuing."

You can find these steps along with other information that may be helpful, here: Use more than one version of macOS on a Mac - Apple Support
If you still have questions, reach out to us directly here as this may require closer guidance: Get Support
Cheers!