Diffrence between $and operator and just comma?

Question 1>
I still can’t clearly tell the difference between using the $and operator and “,”
Can anyone tell me the difference between the three sentences below?

Method1: Didn’t work

db.trips.find({ 
    tripduration: null, tripduration: { $exists: true } 
}); // Doesn't work,

Method2: works!

db.trips.find({
    tripduration: { $exists: true, $eq: null } 
})

Method3: Works!

db.trips.find({
    $and: [{ tripduration: null }, { tripduration: { $exists: true } }] 
})

And if the fields in the query are different, just comma(1st try method) seems to be enough.

db.trips.find({ **//Works!**
    "start station id" : 279, 
    "start station name" : "Peck Slip & Front St",
    "bikeid" : 16118,
})

Is method 2 and method 3 used when field names are same?

Welcome @ys_70859

Here’s an excerpt from the official documentation re this:

By the way, if in any way your post provides answers to an exam or lab question, I would suggest that you redact the answers as it’s against forum guidelines. :+1: