difference between non-_php and _php log files esp. with SSL

I have some specific questions not addressed by the general dox on log files at:

https://docs.webfaction.com/software/general.html

First let's assume only http:// no https:// (no SSL).

We see 2 log files in ~/logs/frontend:

access_WEBSITE    .log.1_2017_11_08 -> front-end nginx log
access_WEBSITE_php.log.1_2017_11_08 -> "Shared Apache" log for Static/CGI/PHP app

QUESTION 1: The dox say that nginx is acting as a "front end." What exactly does "front end" mean? Does it mean nginx is acting as a caching server, and so may sometimes respond to the client directly without requesting anything of the "back end" shared Apache server, and creating no entry in the "back end" log? This is very relevant for the log files. Suppose the client fetches /foo and /foo is an URL that resides on a Static/CGI/PHP app. If nginx is acting as a caching "front-end," then that request for /foo may appear in the non-"_php" log but fail to appear in the "_php" log. Is that possible?

QUESTION 2: If the answer to question 1 is "yes it's possible," it means that the "_php" log is pretty much useless for any traffic analytics purpose, correct?

QUESTION 3: If the answer to question 1 is "no it's not possible," then can you describe any scenario in which the "_php" and non-"_php" log will differ, for a request that maps to a shared Apache server?

QUESTION 4: When would the back-end "_php" log ever be useful?

Now, let's add SSL into the mix.

To support both SSL and non-SSL for our site, we have to create two separate WebFaction "websites". Call them:

WEBSITE
WEBSITE_SSL

This appears to create 4 log files:

access_WEBSITE        .log.1_2017_11_08 
access_WEBSITE    _php.log.1_2017_11_08
access_WEBSITE_SSL    .log.1_2017_11_08
access_WEBSITE_SSL_php.log.1_2017_11_08

I am not surprised to see two different flavors of the non-"_php" log files (the front-end nginx log files).

But I am surprised to see two flavors of the "_php" back-end log files.

I thought that the SSL connections were terminated at the front-end server and that the back-end shared Apache servers always saw http:// requests. That is why, when writing mod_rewrite rules, we had to test SSL using the X-Forwarded-SSL header like:

RewriteCond %{HTTP:X-Forwarded-SSL} !on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [R=302,L]

rather than testing %{HTTPS} as most people seem to do with other ISPs.

QUESTION 5: Can you explain this confusion? How are the front and back end servers actually set up? Are separate (virtual) back-end servers fielding the SSL and non-SSL traffic?