New US visa approved while on travel [closed]

I currently have a H1-B visa which is due to expire in March 2022. My H1-B extension was submitted in June and is currently under review. I am in the US and will be travelling to the UK (sep 17) for a number of weeks on my current visa ( I am able to leave and re enter). What happens if my H1-B visa extension is approved while I am on travel? Should I return to the US before the start date of the extension? Or should I try to get a visa stamp for the extension while I am away? Will the H1-B extension begin from near the expiration date or will it start sooner?