why does my Mac show up when a call comes in onmy iphone

That is Continuity. The following from Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support has instructions on what is required for it to be on:

Use iPhone Cellular Calls with any Mac, iPhone, iPad, or iPod touch that meets the Continuity system requirements. It works when your devices are near each other and set up as follows:

• Each device is signed in to iCloud with the same Apple ID.
• Each device is signed in to FaceTime with the same Apple ID.
• Each device has Wi-Fi turned on.
• Each device is connected to the same network using Wi-Fi or Ethernet.
• On iPhone, go to Settings > Phone > Calls on Other Devices, then turn on Allow Calls on Other Devices.
• On iPad or iPod touch, go to Settings > FaceTime, then turn on Calls from iPhone.
• On Mac, open the FaceTime app, then choose FaceTime > Preferences. Click Settings, then select Calls From iPhone.

If your carrier supports Wi-Fi calling on other devices, you can set up those devices to make and receive calls even when your iPhone isn't turned on or nearby. Learn about Wi-Fi calling.

Do the opposite of what was required to turn it on from my previous reply -- see below but where it says "turn on" turn off instead; in the 3rd item below instead of select, deselect.

• On iPhone, go to Settings > Phone > Calls on Other Devices, then turn on Allow Calls on Other Devices.
• On iPad or iPod touch, go to Settings > FaceTime, then turn on Calls from iPhone.
• On Mac, open the FaceTime app, then choose FaceTime > Preferences. Click Settings, then select Calls From iPhone.