The Enhanced Visual Search in the iOS Photos app sends encrypted, anonymized data from photos to Apple.
Also see Apple's technote (12203) About Enhanced Visual Search in Photos and the Photos & Privacy page.
If any landmark matches are found, Apple's servers then send a match list back to Photos on your device, so it can display a list of the matching photos.
Using EVS in the Photos app, you can type a landmark name in and the app will scan your photos for matching landmarks.
Your actual photos are never sent to Apple, but rather the device uses a machine learning (ML) model to create a vector embedding (subset) of possible landmarks from photo data.
