sean Followed your procedure for migrating a wordpress site:

sean Create a generic PHP-FPM+Apache app on our end and attach it to a site route.
Create a blank MariaDB database and user on our end.
Copy your existing WP into the new PHP-FPM+Apache app.
Adjust your wp-config.php with the database credentials for your new database.
Import your data into the new database.

All worked well, but when I connect to the website index.php shows up on a white background (sorry for backslashes):

<?php
/**
* Front to the WordPress application. This file doesn't do anything, but loads
* wp-blog-header.php which does and tells WordPress to load the theme.
*
* @package WordPress
*/ etcetera etcetera...`

Looked here for clues:

sean _{/logs/app_name/apache_error.txt}

and I found this:

[Thu Feb 20 15:59:52.628146 2020] [core:error] [pid 174052] [client 127.0.0.1:41966] AH00124: Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace., referer: mywebsite.com

It's a multisite, so it has a really standard .htaccess file:

Header set Access-Control-Allow-Origin "*"
<FilesMatch \.php$>
    SetHandler php72-cgi
</FilesMatch>
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>
# END WordPress

Problem is way beyond my capabilities...