That’s pretty much what the questioner said, but let’s be clear, all refers to everything,

p{
    width: 200px;
    height: 200px;
    margin: 300px auto;
    background-color: #336699;
    
    transition: all 1s;
    /*transition: height 1s;*/
}
p:hover{
    width: 300px;
    height: 400px;
}

I think all here refers to the style after the transition. The above code runs and the width and height of p change smoothly. When we change all to height, only the height transformation process has animation effects. width is an instant change.

Yes, all can also be omitted without writing

Yes, it is recommended to specify a certain attribute when using it