Settings>Device Maintenance>Storage>Internal Storage. Then tap on Cached Data and confirm you want to clear it.
The other cache is a bit harder to clear. Power off the phone. While NOT on a charger, hold down the volume up and bixby buttons at the same time and then hold the power button. Hold them all down until you see the blue recovery screen start. In Recovery, use the volume button to scroll down to Wipe Cache Partition. Hit the power button to select it. Scroll down to yes on the next screen and hit the power button. Then hit the power button to select Reboot system now.