How do you move ALL the data from an old drive to a new drive So, I'm going to replace the drive in a mid-2012 macbook pro with an ssd. What would be the easiest way to transfer all the data (files) from the drive to the ssd? Is it possible to have all the programs transfer over also? Do I have to reinstall the OS?
Provided you have an enclosure for the HDD or SSD that you can connect via USB, you could use a tool like Carbon Copy Cloner. (CCC has a 30-day trial and is $39.99 to purchase after that. I've had good experience with CCC, but other free or paid utilities may work better for you.)

The convenient part of using CCC or another utility is that it can make the disk bootable, which is required for swapping the disks like you are asking. If you don't make the disk bootable, it won't boot.

If you don't have an enclosure, you can use an empty external hard drive as a middleman. Use your utility to make a bootable copy of your HDD on your external drive, physically swap out the HDD and SSD, boot to macOS from the external drive, then make a bootable copy from the external drive to the new SSD.

If you don't have an external drive or an enclosure, you may have to reinstall macOS and all your other applications and replace your data.