Modify entity x_movement while keeping z and y movement the same

I'm new to the data command, I don't really understand much of it. I got the entity to change it's movement nbt and move. I used the data merge command. This way the motion NBT of the entity got changed to the given motion. I want it to only change one of the values, but I can't seem to get it to work. How do I make it so only one of the motion values change, while the rest stay the same? Also, different question but also regarding to motion. Is it possible to have random values in NBT/components and how should I go about doing this if it's possible?