I installed nvidia-driver, but nvidia-smi didn't work and I suffered a lot, so I'll write it down here as a memo for myself.
I don't know why, but nvidia-smi may throw an error when Secure boot is enabled. Let's disable it from the BIOS.
Select the one that seems to be just right from the candidates that came out in
sudo apt install nvidia-driver-440
Install as follows.
For some reason, the .run file on nvidia's official website didn't work.