Welcome to Android Central! Apps typically have to download to and install to Internal Storage. Some phones (usually older ones) have the Move to SD feature in the Settings>Apps menu, which only moves a portion of an app to the SD card. The other possibility is if the phone supports Adoptable Storage (i.e., formatting the SD card as Internal Storage, not Portable Storage). If your phone doesn't support either of these options, then you'd have to root the phone and use an app like Link2SD or App Mgr III (and I wouldn't recommend rooting if you're a novice).
That phone has only 8 GB of internal storage, which means the amount available to the user is probably about 4-5 GB (after accounting for the OS and any bloatware). That's very small by today's standards, so a budget phone like this is really only good for very basic use. If you want to install more apps, you need a phone with 16 GB at the bare minimum, and preferably at least 32 GB.