Choosing a screen saver

xscreensaver

unused

xlockmore

xlockmore is gone from Ubuntu so grab and build the latest release

apt install build-essential libx11-dev libxext-dev libxt-dev
wget ...
tar xJf ...
cd ...
./configure
make
make install

usage

xlock

xtrlock

    apt install xautolock xtrlock

usage

xtrlock

Resources

xscreensaver

https://www.jwz.org/xscreensaver/

xlockmore

http://sillycycle.com/xlock/

https://askubuntu.com/questions/1063728/cant-download-xlock-into-ubuntu-18-04

https://logs.paulooi.com/how-to-lock-your-machine-and-display-application-screen.php

https://ubuntuforums.org/showthread.php?t=2225417