The video shows the steps we take after a clean install.

You can type every command or you can put them in a text file and run that one script. The latter is more convenient.
I have a personal github to install elements after every clean install at github.

Here is the content of the script on January 2021.

sudo pacman -S --noconfirm --needed linux-lts 
sudo pacman -S --noconfirm --needed linux-lts-headers
sudo pacman -S --noconfirm --needed optimus-manager-qt
yay -S nvidia-390xx-dkms
yay -S nvidia-390xx-settings
yay -S nvidia-390xx-utils

After installing all the software and making all the smart choices you reboot, activate optimus-manager and select nvidia.