Using the steps below you should be able to do that:
- Create a new attribute for your circle-like polygons (let's call it "Plot Number") in the attribute table.
- Use the "Select by Location" tool (Processing Toolbox -> Vector Selection, with the geometric predicate "are within") and then the Field Calculator in your Attribute Table (don't forget to tick "Only update n selected features" at the top) to give each circle-like polygon its Plot Number (for instance all circle-like polygons within Plot 1 will have "Plot 1" as the Plot Number attribute)
- Use "Dissolve" (Processing Toolbox -> Vector Geometry) with Plot Number as the "Dissolve field(s)" so that your circle-like polygons will be grouped by their Plot Number attribute.
- Now you can select all circle-like polygons by the plot they fall in and do all the calculations you want based on that.

Your final result should look something like the image below.

[![enter image description here][1]][1]


 [1]: https://i.sstatic.net/hqRtK.png