You use the file /etc/fstab
to decide which elements of your filesystem get stored on which storage hardware. In other words, you only get to choose which directories get stored on which disk.
Since many applications install files in multiple locations across your filesystem, you generally don't get to decide on an application-by-application basis. There are a few exceptions to this, like AppImages and Flatpacks.
For most folks it's simplest to migrate your /home
directory to the HDD. It's often the biggest space-consumer anyway. For example, of the 26G used on my 30G SSD, 23% is my /home.