I can only find union through Tools > Union when i have no bodies selected. Instinctually I feel like I should be able to choose the bodies first and then chose to union them?
Either because booleans can’t be applied to your selection (eg. the bodies don’t touch each other, or you only have one body selected), or it’s under the … menu because it doesn’t fit to your screen.
Correct, your last screenshot has faces selected which cannot Union.
There are times that after selecting two bodies that you will find the Boolean operations, including Union, in the … More menu.

