Note: If you're just looking to beautify/improve your inspector, rather than doing it as an actual project/experience, your better off looking for plugins.
The Custom-Editor API that Unity provides are a bunch of Tools, not a House.
You will end up pouring a lot of effort into making your inspector 'look neater'.
If you just want to create a game, use plugins to decorate your inspector.
MyBox is one of the Plugins I use, and recommend.
Now back to the question
I managed to pull it off by using a combination of EditorGUILayout.Foldout and looping through the array size to create multiple EditorGUILayout.IntField.
// True if the user 'opened' up the array on inspector.
private bool countIntsOpened;
public override void OnInspectorGUI() {
var myTarget = (N_USE)target;
myTarget.myTest.name = EditorGUILayout.TextField("see name", myTarget.myTest.name);
myTarget.myTest.countOnly = EditorGUILayout.FloatField("see float", myTarget.myTest.countOnly);
// Create int array field
myTarget.myTest.countInts = IntArrayField("see int[]", ref countIntsOpened, myTarget.myTest.countInts);
}
public int[] IntArrayField(string label, ref bool open, int[] array) {
// Create a foldout
open = EditorGUILayout.Foldout(open, label);
int newSize = array.Length;
// Show values if foldout was opened.
if (open) {
// Int-field to set array size
newSize = EditorGUILayout.IntField("Size", newSize);
newSize = newSize < 0 ? 0 : newSize;
// Creates a spacing between the input for array-size, and the array values.
EditorGUILayout.Space();
// Resize if user input a new array length
if (newSize != array.Length) {
array = ResizeArray(array, newSize);
}
// Make multiple int-fields based on the length given
for (var i = 0; i < newSize; ++i) {
array[i] = EditorGUILayout.IntField($"Value-{i}", array[i]);
}
}
return array;
}
private static T[] ResizeArray<T>(T[] array, int size) {
T[] newArray = new T[size];
for (var i = 0; i < size; i++) {
if (i < array.Length) {
newArray[i] = array[i];
}
}
return newArray;
}

Not as nice or neat as Unity's default. But gets the job done.