Why is this so difficult to do (record set listing)?

Creating something like this on the interface level is pretty simple but I am sure you have a bigger purpose. Provide some more supporting information. 

To add more context.
Working on five different applications.
Each application uses a STAR schema and in the 'Admin' the related tables need CRUD interfaces and process models.
The interface in the current setup with a different LowCode platform has this type of interface.

Plus, there are some areas which are not in the Admin which have a record listing of related child records, which have the interface for doing things like Deleting Selected Records, Merging Selected Records, etc.

The client explicitly stated that they NEED to have the "X/Y records" selected.

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@Harshit Bumb
I take issue with someone saying that creating this "is pretty simple".
It may be easy for someone who has used and/or programmed in Appian for more than a few months.
For instance a heart surgeon may say that doing a by-pass "is pretty simple", but you don't want me to do that operation, even though it "is pretty simple".

Hm... I do not feel that I fully understand about what we are talking here.

In general, GRUD can easily be auto-generated using Records and related actions and views.

Can you go a bit deeper into your goals and design?

I need to recreate this

This type of interface is used anywhere from 10 to 20 times per a site and there are five sites, each time with different fields and data.
Note that on the bottom left corner there is "0 Rows Selected | 56 Rows Displayed".
I need to be able to select one or more rows and click the delete button to delete the selected rows.

But also in the site some times this interface is used to merge two or more records what are similar and deemed duplicates.

so, you wants to make a generic interface which can be used as per your requirements - 
1. data and fields are dynamic
2.selected rows should be deleted once you hit delete button

but I didn't get your last line - 'But also in the site some times this interface is used to merge two or more records what are similar and deemed duplicates.'

can you please explain it ?

Okay, I know that I'll have to recreate this interface 100x or something like that as each table/record type is different. 
Right now I'm having issues creating it one time.

Nine out of ten times I need to have only a 'DELETE' type button, but other times I need to have both a delete button and a separate button which executes a different process, but still has all the values from the selected check boxes.

I created such dynamic interfaces before. If you are interested in a collaboration ...

I created such dynamic interfaces before. If you are interested in a collaboration ...

Thank you for the offer, but do you have any step by step tutorials on how this can be done?
I have found the tutorials docs.appian.com/.../Grid_Tutorial.html
on how to create most of this type of interface, but I'm not getting the record count.

Previously, some one on this site with a title of 'Certified Appian Developer' or something like that said something along the lines of 'Data Store Entity is the old style and should not be used'. 
Maybe they were wrong in saying that, but before they said that I was trying things out with Data Store Entity and I was having issues with that as well.
There appears to be two or more ways to query the data (expression rule, inline) with different data types (record type, data store entity) and some examples may not use the Appian Best Practices.

Level one certified Appian, that can be accomplished with a good memory and studying, which may not translate into actual business solutions. 

I find that in C# or Java (SpringBoot) or most other environments creating something like this can be done relatively easy, but you may need to know Razor or ThymeLeaf or go all out with React.JS.

From my perspective, Appian is NOT a UI building toolkit. It is made to implement applications driven by business processes.

Using it to built this kind of catch-all-super-dynamic interfaces takes a lot of experience and know how.

If this is what you are looking for, I know how to do that:

if you are looking for this ->

a!localVariables(
  /*/make sure ri!data is notnull*/
  local!fields: a!keys(index(ri!data, 1, "")),
  local!selection,
  local!selectedRows,
  {
    a!gridField(
      label: "Performance Review Schedule",
      labelPosition: "ABOVE",
      data: ri!data,
      columns: {
        a!forEach(
          items: local!fields,
          expression: a!gridColumn(
            label: fv!item,
            value: index(fv!row, fv!item, "")
          )
        )
      },
      pagesize: 10,
      selectable: true,
      selectionValue: local!selection,
      selectionSaveInto: {
        local!selection,
        a!save(
          local!selectedRows,
          append(local!selectedRows, fv!selectedRows)
        ),
        a!save(
          local!selectedRows,
          difference(local!selectedRows, fv!deselectedRows)
        )
      }
    ),
    a!richTextDisplayField(
      labelPosition: "COLLAPSED",
      value: {
        a!richTextItem(
          text: if(
            a!isNullOrEmpty(local!selectedRows),
            "0/" & length(ri!data) & " records selected",
            length(local!selectedRows) & "/" & length(ri!data) & " records selected"
          )
        )
      }
    ),
    a!buttonArrayLayout(
      buttons: {
        a!buttonWidget(
          label: "Delete",
          icon: "trash-solid",
          style: "DESTRUCTIVE",
          disabled: a!isNullOrEmpty(local!selectedRows)/*saveInto: here is logic for delete*/
          
        ),
        a!buttonWidget(
          label: "seprate",
          icon: "code",
          style: "NORMAL",
          /*saveInto: here is logic for seprate*/
          showWhen: not(or(ri!isOnlyDelete)),
          disabled: a!isNullOrEmpty(local!selectedRows)
        )
      },
      align: "START"
    )
  }
)

That does look like what I'm after Amaan Shekh