Datum-Data difference in map behavior in d3

I'm pretty new to d3js and trying to understand the difference between using data and datum to attach data to elements. I've done a fair bit of reading the material online and I think I theoretically understand what's going on but I still lack an intuitive understanding. Specifically, I have a case where I'm creating a map using topojson. I'm using d3js v7.

In the first instance, I have the following code to create the map within a div (assume height, width, projection etc. setup correctly):

var svg = d3.select("div#map").append("svg")
    .attr("width", width)
    .attr("height", height)
    .attr("transform", "translate(" + 15 + "," + 0 + ")"); 

var path = d3.geoPath()
          .projection(projection);

var mapGroup = svg.append("g");

d3.json("json/world-110m.json").then(function(world){
  console.log(topojson.feature(world, world.objects.land))

  mapGroup.append("path") 
     .datum(topojson.feature(world, world.objects.land))
     .attr("class", "land") 
     .attr("d", path); 

});

The console log for the topojson feature looks like this:

And the map comes out fine (with styling specified in a css file):

But if I change datum to data, the map disappears. I'm trying to improve my understanding of how this is working and I'm struggling a little bit after having read what I can find online. Can someone explain the difference between data and datum as used in this case and why one works and the other doesn't?

Thanks for your help!

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