At first my son would smile mainly when he had gas or in his sleep and now he smiles when we play with him, when he sees himself in the mirror, or when I make certain noises at him. He doesn't really react with smiling just by seeing my face yet I have to play with him and stuff. The first time he smiled he was 5 weeks old and it was because I showed him himself in a little baby toy that had a mirror. Also now if I make an "oooo" noise at him, he will mimic the noise and smile. I read that it's really good for them to help develop their motor skills to do that because he hears the noise he is making when I repeat it and it teaches him to make more noises and talk.
My daughter does both. She automatically smiles if i say stinky or even just looking at our dogs can make her have a reaction. sometimes i catch her talking to herself in the middle of the night and she cracks herself ip
My daughter started laughing about 3 months but even just a month later her laugh is changing. It starts as a weird noise and slowly starts to sound like a real laugh
My son started smiling around 5 weeks, and he hasn't laughed yet. He's 10 weeks old now. I read online anywhere from like 3-6 months they will start to laugh but I think usually it's more toward 3/4 months