If I remember correctly, if you go into settings on steam in both your pc and steam deck and set them to share the downloaded games. Then just click install as per normal on your deck and instead of downloading it will automatically drag the game from Your pc. I’m sorry I can’t remember exactly but it was really simple to do and I only did it once some months ago. I only managed 36mbps over my home network but still considerably quicker than my poxy home internet