How to move a presentation from PowerPoint to Keynote

Keynote usually does a pretty good job of importing PowerPoint presentations., Just File -> Open and choose the .pptx file

What's often missed are differences in transitions where PowerPoint and Keynote support different effects.

A quick walkthrough of your presentation should highlight these and you can update the Keynote version with any needed changes.

Open the PowerPoint presentation with Keynote and it will translate what it can into a Keynote presentation document without changing the original PowerPoint document.

See what survives as Keynote is not a PowerPoint clone, and then refer to Keynote User Guide for Mac - Apple Support