1- Create a Classic report with a normal query (including a key column, like empno in this case):
select empno,
name,
job,
hiredate
from emp
2- Select the column that will serve as a button:

3- Go to Column Formatting -> HTML Expression on the attributes display region:
and paste this code:
<button class="t-Button " id="#EMPNO#" onclick="showRow(#EMPNO#);" type="button"><span class="t-Button-label"> + </span></button>

4- This will generate a button associated to each "empno", notice that I'm setting the id of the button to #EMPNO# (being the value of empno to that row) and I'm setting an onclick call to a javascript function called showRow(empno). In this function is where you'd call either a new page passing the parameters needed or show a modal popup whit the fetched data to beeing modified.
Here's a working example