The only liquid would be an electrolytic capacitor failing, there just isn’t any in your system. It’s more likely a stain created during the the manufacturing process.
As to why your system keyboard & trackpad have stopped working will need some deeper diagnostics. Do you have a bluetooth keyboard and mouse which you can sue with your MacBook? If you can see if you can check the USB section in the System Report, do you see the hub and the internal keyboard and trackpad?