placing an image that's the same size as the document page size but somehow got half of the ppi

The problem is, I couldn’t switch from pixel to inch under the current document setup intent: web

You don’t have to change a document’s Intent in order to change ruler units—choose View>Show Rulers and Control-Click on the rulers.

From there you can set the current Ruler Units for the document’s horizontal and/or vertical rulers to Points, Picas, Inches, Millimeters, Centimeters, Ciceros, Agates, Pixels. Pixel units are static and equal 1/72”.

The current Ruler Units also affect many of the menu and palette unit displays.

​@leo.r is correct - just to elaborate a bit

The confusing part is that InDesign pixels are not the same thing as Photoshop image pixels.

Your image is 1600 × 2400 px at 150 ppi, so its physical size is about 10.67 × 16 inches.

But an InDesign document set to 1600 × 2400 px is effectively much larger physically, because InDesign’s pixel unit is based around 72 units per inch. So a 1600 px wide InDesign page is about 22.22 inches wide.

That means when you fit your 150 ppi image to that page/frame, InDesign is scaling it to around 200%. Once a 150 ppi image is used at 200%, the effective resolution becomes about 75 ppi.

So nothing is wrong with the image. The Actual PPI stays at 150 because that is the image’s own metadata. The Effective PPI drops to 75 because the image is being enlarged in the layout.

As Leo says - for predictable results, either work in physical units, or make the image 72 ppi if you want its pixel dimensions to match an InDesign pixel-based document without scaling.