How and what exactly do I look for remote access programs or similar?

Did you check Event Viewer logs? Windows automatically audits logon events and logs every time you log into your device. Event Viewer will provide a long list of activities.

• Press the Windows Key on your keyboard or in the Search Box, type: eventvwr and click on the top search result.
• Press Enter to open.
• Go to Windows Logs > Security and look for any suspicious logon attempts. 
  4624 records as a Logon.
  4672 records a Special Logon.
  4634 will be listed when an account logs.

Do you have Remote Access Enabled? If so, disable it to prevent unauthorized users from connecting to your computer.

• Press the Windows Key on your keyboard or in the Search Box, type: SystemPropertiesRemote.exe (or sysdm.cpl) and click on the top search result
• Press Enter to access System Properties.
• In the Remote tab, ensure “Don’t allow remote connections to this computer” is selected.

How To Check For Unauthorized Access To Your PC with Event Viewer

Related Resources:

• How To Check If Someone Is Remotely Accessing Your Compute
• How to See Who Logged Into a Computer (and When)
• Signs to Tell If Someone Is Remotely Accessing Your Computer
• How to See Who Is Logged Into a Remote Computer
• How To Track Windows Computer and User Activity

If Microsoft Defender Antivirus and Defender Offline cannot find any threats or finds but cannot remove threats, there are various secondary opinion scanners which can be used to supplement your existing antivirus.

• Emsisoft Emergency Kit (EEK)
• Malwarebytes Anti-Malware
• Sophos Scan & Clean
• AdwCleaner

I provide a more detailed list of malware removal tools with scanning instructions in Supplementing your Anti-Virus Program with Anti-Malware Tools (Post #4)...just scroll down near the bottom of that post.