I believe this great article explains what you want, with screenshots galore. You want to create a special session in the login manager that will start a browser like Google Chrome using the kiosk option. You can set it up so that normal users automatically log into this kiosk session, but introduce a short time delay to allow an admin user to log into the normal Ubuntu session to administer changes to the system.
The steps are fairly detailed and so I won't reproduce them in their entirety here. But since Ask Ubuntu discourages just linking (in case the linked site goes down), I will cut and paste here the key steps. You should go to the link for the full instructions.
The desktop file you need to create in
/usr/share/xsessions/ should have:
Comment=Chromium Kiosk Mode
chromeKiosk.sh should be placed in the same directory with execute permissions and look like:
xscreensaver -nosplash &
cat ~/.config/chromium/Local\ State | perl -pe "s/\"bottom.*/\"bottom\": $(xrandr | grep \* | cut -d' ' -f4 | cut -d'x' -f2),/" > ~/.config/chromium/Local\ State
cat ~/.config/chromium/Local\ State | perl -pe "s/\"right.*/\"right\": $(xrandr | grep \* | cut -d' ' -f4 | cut -d'x' -f1),/" > ~/.config/chromium/Local\ State
while true; do chromium-browser %u --start-maximized; sleep 5s; done