How to move a presentation from PowerPoint to Keynote

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

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.