How to manually download the expansion (obb) files for your app on play store?

I have published my app on play store with expansion files (obb format). On play store it the download size is apk size + obb file size. Can I get answers for following queries?

1. Is there any settings on console to download apk only and then manually trigger to download apk?
2. If you add expansion file, is it automatically downloaded with apk?
3. Sometimes it does not download automatically, what is the reason for that?

I was looking to download the app first and once app is installed, manually trigger the download of the expansion files (obb) from the play store.

After testing and speaking to customer care of Google developlers Play console I got this specification : Google play automatically downloads the expansion files while downloading the apk most of the times. There are exceptions sometimes it does not download with the apk and need to trigger download manually. One of the problem can be limited storage on the device.Then it needs to be triggered manually. The size of the apk reflects the size of the apk and size of the expansion file together.