If you are working with the AGOL version of WAB then customization is not a possibility. You will have to use WAB Dev if you are going to do any customization.
What you want to do is very possible but will likely be difficult for someone with no JS experience. The attribute widget does listen for selections that occur on a layer (so it does respond to selections using the select widget) and the AT widget does have the ability to export to a csv. So there may be no need for any custom development.
The space where you will want to look is: Web AppBuilder Custom Widgets
I will include 2 resources compiled by Rebecca Strauch, GISP that are very valuable: