When you hold the Option button at startup your Mac should look at external USB devices for a qualified version of macOS. The USB drive must have a bootable version of macOS installed. If the USB device has no macOS properly installed, or has a version that is too old or too new, then the Mac will ignore it and boot normally from its internal drive.
A qualified version of macOS is one that your Mac can run. A Mac cannot be booted to a version of macOS that predates its own release.
The macOS version you can upgrade to depends on the year your Mac was released.
Click the Apple menu () at the top of your screen and choose About This Mac.
✅ Check the compatibility of your Mac hardware with newer versions of macOS software:
➡️ Macs compatible with macOS 10.13 High Sierra (c.2009)
➡️ Macs compatible with macOS 10.14 Mojave (c.2012)
➡️ Macs compatible with macOS 10.15 Catalina (c.2012)
➡️ Macs compatible with macOS 11 Big Sur (c.2013)
➡️ Macs compatible with macOS 12 Monterey (c.2015)
➡️ Macs compatible with macOS 13 Ventura (c.2017)
➡️ Macs compatible with macOS 14 Sonoma (c.2018)