Home Assistant can integrate the energy usage of individual devices into Home Assistant. That way you can see the impact of individual devices on your total energy consumption.
Smart plugs sit between the device and the outlet and measure the energy flowing through the device.
Depending on what protocols you use at home, you can use Zigbee, Z-Wave or Wi-Fi based plugs.
Smart relays sit behind your “normal” switches and make them smart. It allows you to control the devices via Home Assistant and via the connected buttons/switches.
Some smart devices, such as air conditioning, boilers, and others, may provide a power sensor, measured in Watts. You can use the Integration (Riemann sum integral) integration to calculate the energy your device is using. You can then use the energy sensor in the Energy Dashboard, as individual devices.