replace HDD with SDD in 2012 macbook pro
How do I
install a new SSD without OS in macbook pro 2012
MacBook Pro 13″, macOS 12.2
You open the Mac up, remove the existing HDD, and plug in the new SSD in its place.
With that done, close the Mac up, turn it on, and hold down the option + r keys to enter Recovery. From there you can install the OS onto the new hard drive.
click here ➡️ How to reinstall macOS - Apple Support