wi fi – Something in the background takes a big chunk of my Wi-Fi network after a classmate has picked up my phone

A little story back:
So this is what happened today afternoon (around noon). One of my classmates picked up my phone to find out about its features and content. He spent 8 to 10 minutes with him. I was not too careful so do not be rude. After returning home in the evening (around 17:00), I noticed a slight shift (a few times). Now (21:00), something is taking my WIFI speed while the speed indicators in the status bars are displayed.
What I did:
Tried "Force off" on the Photos application because it sometimes starts backup, checked that no Whatsapp backup is in progress. You searched the Internet for "which app uses my wifi" and all results are related to "who uses your wifi" or "data usage on mobile data", but I do not have a data plan mobile. I only use WIFI.
Ok I think I am very clear. The question is What is WIFI on my device in the background? PLEASE HELP CAUSE I THINK I AM FINDING ME