I have a scene where I need to have some objects rendered semi-transparent (say, the four machines with red parts from the screenshot) . How do I do that? Do I need to duplicate all the used materials and make them transparent in Nodes? Or is there an easier way?
Just move the transmission slider up in the principled bsdf node until its transparent enough for you. Also move the roughness slider for a clearer material.
Actually, you don’t have to duplicate any materials. In blender you can keyframe almost any property (even the renderability toggles). When you right click most value inputs you are given the option to keyframe that property:
It looks like you are using eevee, so set the blend mode setting on the material to alpha blend first or transparency won’t work.
If you need the material to “jump” to the semi-transparent state for an animation or something then you’ll need to open the graph editor and set the interpolation type to “constant” (press T to get the interpolation options):
Since this method works for most inputs, it can be used to animate the “fac” attribute of mix shaders allowing you to blend between two different materials if you want: