How do I connect my imac (late 2015) to macbook pro 2019 as a dual display?

Good day nell,

Target Display Mode was not supported with 2015 iMacs, and even if you had a Mac that would support it, recent system functionality is spotty in support of that.

Use your iMac as a display with Target Display Mode - Apple Support - https://support.apple.com/HT204592 - iMacs late 2009-Mid 2014

This could only be done with a very few and very specific older iMacs and support was very limited. Read this topic for a 2021 perspective on this: https://discussions.apple.com/thread/252271436?answerId=254300142022#254300142022

and this 2021 post by Allan Jones about software limitations:

https://discussions.apple.com/thread/252344497?answerId=254428494022#254428494022

This is not the same but read about Screen sharing https://support.apple.com/guide/mac-help/share-the-screen-of-another-mac-mh14066/mac