Hoe can I use my iMac as a display for my MacBook

Please read Target Display Mode (Target Display Mode) to see if you can or not. My recommendation though is rather than buying expensive cables for a difficult setup and unreliable solution to simply buy an external display for the MB. Remember Macs are computers they are NOT displays for other devices.

To clarify, Mid 2014 iMacs were the LAST to be able to act as a monitor for another Mac, and could run no higher macOS than 10.13 High Sierra. Your Late 2014 was never supported with TDM, regardless of its macOS version.

There are only a few models of iMac that can do this. If your iMac is from mid 2014 there is a chance that it will work, but not with older models. Please check this link for reference and instructions

Use your iMac as a display with target display mode - Apple Support