called XSLT List View Web Part, but when I go to "Insert Web Part" in
my Page, I don't see that web part listed.
"Insert Web Part" option lists already published (deployed) to Shaqrepoint webparts.
XSLT List View Web Part is for being created designed dynamically based on a data source content.
You can do it from browser but if you do it from Sharepoint Designer 21010 (SPD), you will clearly see its markup or tag XsltListViewWebPart as well as its code.
So, in SPD create a WebPart page (Site Pages > there are many ways how to further do it, quite obvious.
There, in Design view, in a placeholder I usually insert a few line breaks in order to be more comfortable with further editing.
Then click Insert tab, on ribbon > DataView > choose a (document) library under Documents and Libraries section of dropdown list of choices.
Then, you will clearly see WebPartPages:XsltListViewWebPart
You can repeat it inserting other XsltListViewWebPart web part.
Though, answering your title question, you most probably will want to make first a specific/separate data source.
IMO, Sharepoint is good for simple OOTB gathering of data but not its (or "their, I am not good in English) flexible (or advanced) management (and presentation). If you want to manage efficiently and dynamically, on the fly, (large amount of) data then you most probably should use Database Management System like MS SQL Server, here are some keywords from MSSQL:
- MS BI - Business Intelligence
- SSAS - MS SQL Server Analysis Services,
- SSRS - MS SQL Server Reporting Services
Update:
Now, after answering, I've noticed that the question is tagged with "infopath", though, IMO, Infopath is quite unrelated and divergent to XSLT List View Web Part usage
I want to gather the data from those completed forms (from every
subsite)
You should promote fields to sharepoint fields during form template publishing in order to be able to easily access the same data from sharepoint fields.
In Infopath Designer, you create data connections/sources to sharepoint (form or doument) library or list, insert controls for fields and links them (with Xpath) to whatever field data from whatever source you wish in any combination.