This post recoreded how I connected to my raspberry pi from a Ubuntu PC via ethernet. The raspberry pi is installed with the Raspberry Pi OS (Jan 2021). I spend embarransingly long time to figure it out.
SSH
For a new raspeberry Pi (3B), I enable all of its interfaces in the configuration. Then I input the following command.
ssh pi@raspberrypi.local
I get some warning, and following the warning, I did
ssh-keygen -f "/home/yushi/.ssh/known_hosts" -R "raspberrypi.local"
Then I did the same command again.
ssh pi@raspberrypi.local
which would require the password on the chip (raspberry by default). After that, the ssh connection is established!
VNC Viewer
It is sometimes convenient to take a look at the desktop of the raspberry pi. I find it very convenient when switching between WIFI networks. The very convenient software that helps is the VNC viewer. I installed it on my Ubuntu PC. And I started a new connection via File -> New connection.... In the VNC Server entry, I entered raspberrypi.local. Then I clicked the new connection, inputed the username (pi) and the password (raspberry), it worked!