I need to find a spy app that somebody somehow installed on my phone,

Hi, Thanks for posting in Google  help forums,

Open the link,

Tap / Click on the picture at the top right,

Select Account,

You can see All Devices currently Signed in and Signed out  for past 28 days for this Account, 

Next,

Select Manage Devices>

Select Device , 

Select  Sign out for Remove unwanted Devices,

Next,

Open the link,

Click / Tap on the picture at the top right,

Select Account in case of multiple accounts,

Select Password> Confirm Sign in>

Enter New password> confirm password> change password.

Wait a minute , Restart device, Sign back into this Account.

You will stay Signed out in All  other Devices.

Still problem persists ,

Reset Device to Factory Settings  and set-upas a New Device ,

Check Sync settings,

Next,

Backup Data,

Next,

Save  / Backup your Passwords /  Data / Files / Photos / Videos  / Contacts / X/ X/ into Google Account,

Next,

Open the link,

https://myaccount.google.com/

Switch between accounts and,

Confirm Default Account for this Device,

Next,

Clear Data in Google play services,

Remove All  Accounts,

First of all remove Non default Accounts,

Finally remove Default Account,

Next,

Reset Device to Factory default Settings,

Setup Device  " as New " while  the Reset , 

( Third party Apps will not be Downloaded and installed automatically)

Setup initially with Default Apps only,

Check the problem persist,

If you have upgraded Android phone OS version  ,It will not be degraded due to Reset ,

After finishing,

Select Automatically Sync Data On and Check successful syncing in Apps,

Next, 

Backup Data,

Next,

Open Google Play store and Install or not Install  Third party Apps,

BE AWARE  of install Apps out of Google Play Store,