create a button on the grid, button must call a microflow with an input parameter of the entity displayed in the grid. Make it the default button et voila you have a microflow with the selected row as an input parameter.
You can only pass on objects to microflows triggered from the client side. This means the data you want to pass on needs to be stored in an object (database or client). Set a dataview in the page, make sure there is an object when you open the page. Give this object all attributes that you wish to pass on and simply use the value of these attributes for text labels. Whenever you trigger a microflow from within this dataview, you can pass on the object as parameter.
Sytze de Bok
I think you have a couple of options
try out the Pivot Table widget – there is a demo project here so you can see how it works
create a separate entity in your domain model to hold the accumulated data, create a microflow to update that data and then before you open the page, call this microflow to update the numbers (so basically a roll your own pivot table approach)
Let me know if you have questions about either approach,