Hi @bgranger ,
By default, Android downloads to the phone's internal memory.
You may be able to move the app to the SD card once it has been installed in the phone by going to Settings > Apps swipe to Downloaded then scroll down to find the app.
Tap on the app and check if there is a Move to SD card option available. If there is tap it and then follow the prompts.
Be aware that not all apps can be moved this way. It depends if the app's developer has enabled this when the app was first developed.
You can also clear space in the phone's internal memory by shifting your data (e.g. pictures, music etc) from the phone's internal memory to the SD Card.
To make the SD card the default write disk go to Settings > Storage > default write disk and select SD card.
This will only affect your data after you have enabled it. Anything that is already in the phone's internal storage will have to be manually moved over using the File Manager app.
be sure to only move your data and not any system files as this could cause the phone to stop working