Unity activity player run in backround -- 2

I made an andriod app using unity and i want to make a service to run in the background. Unity player activity Hello . i made an andriod app in unity . and i want to know the steps on how to make a service run in the background. Mainly audio running in background .Thanks I already made a code and it worked . but i need to know the legit way to do it . i need to know how to make a service run in the background so its not all done in andriod only. We made the app in unity then we export it to android studio so now its a unity app in android studio wich has UnityPlayerActivity i think we need to expand the UnityPlayerActivity before adding or making any class Do you know how to do this? What times are you available to chat . im available from 10 am to 10 pm Louisiana USA time.