Ok. I have now attached two images of the remote. In one of them you can see a red and a black wire. These wires are connected to a relay to "press the button". In the other picture you can see the remote with the buttons removed.
The thing is that I would like to use a transistor instead of a relay. I would like to connect the red and the black wire to the transistors collector and emitter.
OK, I don't give you credit for the photography - better to take the thing out into bright daylight and use a proper camera either with its "macro" function, or at least a metre away and with its maximum zoom to get a perfectly focused photograph.
Nevertheless, you appear to have identified that one terminal of each switch is indeed connected to the negative of the battery. In that case, yes, a simple general-purpose NPN transistor can be connected with its emitter to that negative terminal, its collector to the corresponding switch terminal, and its base through a 1k resistor to the control output of the Arduino given that the Arduino ground is connected to the battery negative of the remote.
Yes, simple and effective.