How to render ground reflections without rendering the ground? I'd like to create a product shot like the following images for a client:


I'm relatively new to Blender, I don't know that much about render passes.
What I would like to do is render the bottle, with the reflection with a transparent background. I do'nt know if this is possible, if its not, maybe there is another way to get this result. Please keep in mind that I'm new to this program.
Not an answer strictly... but...

Use a mirror modifier:

Works for Cycles and Eevee ; )
You can improve it for instance using UV parameters of the modifier.
If you shift its U to 1:

that can give a clue that you can use in the shader, to obtain this for instance:

to use it you can make a node group that you can plug into the alpha input of the principled BSDF (node group as you may have several materials).

The top line uses UV X coordinate (U) to look if under 1 (the mirrored part is over 1 because of the parameter in the modifier).
The bottom line uses Z coordinate to tune a progressive value from 1 to 0 as Z goes down.