Pardon ... difference ... Azure is your virtualization. It host a Ubuntu linux which should be running a distro for Azure .. not other Ubuntu generic distro's.
To make things more confusing - PHP - the php folks, any distro, don't have a driver for mariadb ... matter of fact, MariaDB began it's life by a former employee of MySQL ... who is now owned by Oracle. Fear of Oracle pulling the plug on MySQL has led many linux distro's to 'favor' MariaDB. So the php driver is really the same one as for MySQL ... that's why setting moodle to use mysqli works.
In your ubuntu, one can see this claimed drop in replacement by the command line utilities for MariaDB ... like to make a dump of your database for moodle ... will use mysqldump command.
There is no mariadbdump command. The command to connect to the mariadb server is mysql.
Ubuntu itself now has a special distro for running under Azure.
So going Azure route means 'special considerations' .... and I'll be so bold as to say not very many hosting providers offer Azure for customers.
So your comment ... "hope for an answer (a rare thing to happen)" is probably very accurate.
And, in going forward, community support forums (these forums) will have fewer folks who run your combo - choice of poison.
At any rate, you will have to concentrate your efforts *IN* the guest OS - Ubuntu ... and not necessarily in Azure alone! Having said all that ...
Good luck going forward!