In iOS and macOS, there is a built-in object and face identification component. In addition to being able to identify what a photo or video depicts, it can detect human faces, and it's quite good. If you help it identify people in some photos, it will automatically recognize them in others too.
To start identifying people:
Open the Photos app.
Find a photo with one or more faces and open it.
Swipe up to reveal identified faces.
Tap a face to open that persons photo gallery.
Tap the ”Add Name” tag at the top of the screen and write first and last name of the person in the photo. If the name is already in your Contacts or is identified before, the name will show up when you start to write.
Repeat step 2 to 5 for every person you want iOS to recognize.
Automatic identification of faces is made in the background when your device is connected to a charger and isn't used. If you have a lot of photos, it may take days for iOS to identify all faces. My library of more than 120,000 photos took weeks for it to go through. It has identified more than 100 different people.
To find people already identified, you can either use search in iOS Photos or open the People folder in the Albums tab.
There are lots more to say about iOS face identification, but this should at least get you started. If you have a Mac connected to iCloud with the same Apple ID, faces will be synced across devices.