Hi Ditto - so what I'd recommend is a process more like the following - if you kind of try to keep the pieces that you want to union in the end start out as a simple big piece that then gets cut it will make it easier to have the parts all be connecting together with each other accurately in the end.

To start with get the wheel and the scoop as a full solid piece like this:




Now select the wheel and run Construct > Boolean > Difference, and set the "Keep objects" checkbox, and pick the spout as the cutting object.

The spout part will remain and the wheel will have a hole cut in it like so (spout hidden here to show you):




Now select the spout and run Construct > Boolean > Difference again, and choose the inner curve of the wheel as the cutting object:



That will slice the spout making it flush with that inside cylindrical surface:



Go to the spout piece and select the top and side faces like this:



Then run Construct > Offset > Shell - that will hollow out the spout part like so:



Now you can select the spout and the wheel:



And then use Construct > Boolean > Union to union them together - because they shared a full surface in common on the inside cylindrical area, that will actually get fused together into a single surface there and you'll get this result:




So note there that this process builds pieces that are initially just sticking through each other and then slices them to a common cut - that's generally the best way to make sure that pieces will be matching up properly rather than trying to build 2 pieces totally separately and then trying to position them into place, when they need to be flush where they meet.

Hope this helps!

- Michael