Optimize storage on an iOS device. When or how do the full resolution copies get removed

I have been using iCloud photo library for a week and have been using it to look at some old photos and watch old videos from the photos app on an iPhone. After vieiwing this old data the library now takes up more space on the phone. I have optimize storage turned on.

I assume the iPhone is just storing thumbnails of everything in the library, and when accessing a file it then downloads the full resolution copy (or a higher resolution copy fit for viewing on an iPhone). My question is when does this full or higher resolution copy "return to the iCloud" so to speak, so you are just left with the thumbnail? Or can this be done manually?

For example say I take a 1GB video on my phone today and it gets uploaded to the iCloud photo library today, when will the full resolution copy dissapear from the iPhone and just the thumbnail or lower resolution copy is left taking up space on the phone?

I have a 70GB library, the amount of space taken up on the phone for photo library was around 5GB a week ago, after a week of accessing some of this old data it's now around 15GB

Many thanks