It will depend on how the game is packaged. On your two examples that let you play before finishing download the clients will have multiple downloads that will prioritize everything needed to play early content. This will allow you to play for a bit and by the time you are far enough into the game the rest should have downloaded.

I am not entirely sure how steam manages downloads but I suspect it compresses the whole game as a single download and decompresses and installs it once the download is done. This would mean that nothing is playable until the entire game has been downloaded. It’s something they could potentially change with their download process but not many developers would take advantage of it so it’s likely a low priority thing.