"Filtering" in JSONata is what Paul is demonstrating with this part of the expression -- payload[fitting="wall"]
but since he wrapped it inside the $count(...array...) function, the result is the number "1".
If you remove that you will see the output is just an array of the matching object(s) themselves. If by grouping you also mean "aggregating" or reducing (turning N objects into 1), then yes that is also possible using functions like these.
The best way to see this in action is to copy your data and paste it into the left-hand window on the try.jsonata.org site... then, as you change the expression in the upper-right window, you can see the results (or errors) change in the lower-right.
If you've not read the docs on jsonata.org I would hightly recommend it -- it will take you only an afternoon to get proficient with the syntax, and can save hours of JS coding, if you do much restructuring of data.
