Cutting, for example - drilling holes) in a converted stl is not a big issue.
Adding geometry might not be a big issue.
But changing the size of existing geometry can be a big issue as your part consists of over 157 thousand individual faces.
If you were expecting a feature tree - you are only going to get a "dumb solid".
For freeform editing of stl mesh you can use another product - Mesh Mixer - but for the types of geometry that you posted it is far easier to remodel with primitives like cylinders and boxes. Simple Revolves and Extrudes.
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