How can I filter this report with user name?

No worries, feel free to ask as many questions as you want. As you are bit confused in identifying the field name, I would like to rephrase my explanation as follows:

First of all, execute the step - 1 as specified above. This returns you an output. Now observe the value of columnConfigs in this output.
This columnConfigs is nothing but the array of the columns in your portal report. If we look at the Appian report, we will be able to say what column does what. But when a!queryProcessAnalytics gets us the data, it helps us in identifying the each column by providing the attributes of each column(name of the column, field of the column etc) in the columnConfigs array.

So every column in the portal report is identified by a!queryProcessAnalytics as c1, c2... and so on.

That's the reason why we use the same in queryFilter as well. Similarly in order to access a column in final output, for ex. we read a field 'c1' in the final output as index(<datasubset_variable>.data,"c1",null).

No worries, feel free to ask as many questions as you want. As you are bit confused in identifying the field name, I would like to rephrase my explanation as follows:

First of all, execute the step - 1 as specified above. This returns you an output. Now observe the value of columnConfigs in this output.
This columnConfigs is nothing but the array of the columns in your portal report. If we look at the Appian report, we will be able to say what column does what. But when a!queryProcessAnalytics gets us the data, it helps us in identifying the each column by providing the attributes of each column(name of the column, field of the column etc) in the columnConfigs array.

So every column in the portal report is identified by a!queryProcessAnalytics as c1, c2... and so on.

That's the reason why we use the same in queryFilter as well. Similarly in order to access a column in final output, for ex. we read a field 'c1' in the final output as index(<datasubset_variable>.data,"c1",null).