creating a registration system with CI4, and why is CI "better"?

I get this with php -v in console:

Code:

PHP 7.0.33-0ubuntu0.16.04.7 (cli) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
    with the ionCube PHP Loader + ionCube24 v10.4.0, Copyright (c) 2002-2020, by ionCube Ltd.
    with Zend OPcache v7.0.33-0ubuntu0.16.04.7, Copyright (c) 1999-2017, by Zend Technologies

I tried the apt-get install and got:

Code:

condor1310:/usr/bin# sudo apt-get install php7.3
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php7.3
E: Couldn't find any package by glob 'php7.3'
E: Couldn't find any package by regex 'php7.3'
condor1310:/usr/bin# sudo apt-get install php7.3.22-intl
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php7.3.22-intl
E: Couldn't find any package by glob 'php7.3.22-intl'
E: Couldn't find any package by regex 'php7.3.22-intl'
condor1310:/usr/bin# apt-get install php7.3-intl
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php7.3-intl
E: Couldn't find any package by glob 'php7.3-intl'
E: Couldn't find any package by regex 'php7.3-intl'

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Upgrade your php version to 7.2+ or higher.

Also you need to edit your php.ini file and remove the ; from the intl extension.

Having problems with the upgrade.  Can you assist on which command to use?  I'll also try the php.ini suggestion. Thanks!