A common surprise for homeowners: tile and grout let water through. Grout is porous, and in a shower that's wet every day, moisture works its way behind the tile. What stops it from reaching the studs is the waterproofing layer underneath.
In a properly built shower, that layer is a continuous membrane over the walls and the base, with every seam, corner, and pipe penetration sealed. The shower base is sloped to the drain so water never sits. Niches get waterproofed and sloped too, so they don't collect water.
Old showers built on plain drywall or with tile set directly on a mortar bed without a membrane are the ones that fail. The signs are loose tiles, grout that keeps cracking, a musty smell, or stains on the ceiling below.
When we rebuild a shower, we take it back to the studs, install the membrane, flood-test the base where it applies, and only then start tiling. It adds a day to the job and decades to the shower.