Closing VPN app because battery drainage?! iOS

Hi guys, I hope somebody knows this.

I just got my first commercial vpn and I’m pretty happy with its performance. What I’m not happy with, is the battery drainage. 39% of my overall usage was the vpns app alone. I then went ahead and closed the app after getting connected. Now a few days later it went down to 3%.
Why would I need to keep the app open? iPhone tells me vpn is still connected even with the app closed. What difference does it make? What is the app doing in the background that kills my battery this fast?

Install another vpn, see if anything changes

Install it using iOS’ built-in wireguard configuration and don’t use any external app