The apk is only the installation file, so that can be saved onto the SD card.
If what you are asking about is the actual app installation itself, then that will install itself into the internal storage. That doesn't matter if you install via the Play store or from an apk file. The only way to change that is by rooting and making changes to the system. After installation, some apps will give you the option to move to the SD card, but that depends on the developer and the device. Even if you are given that option, only part of the data may be moved and the actual space savings on the internal storage can vary. There's also stability issues to be concerned with, making it less than ideal to move apps in the first place.