Htaccess rewriterule, file was not found on this server - .htaccess

Question

I am new to the whole htaccess language, but I feel like I am close to what I want.
Facts:
parts is a real folder/directory
anotherpart is a real folder/directory
there are no other files in <root> other then index.php
there are no files in the parts folder/directory
Folder/Directory structure:
<root>/index.php
<root>/parts/anotherpart/this.php
What .htacces I have running:
RewriteEngine On
RewriteBase /
RewriteRule ^parts/anotherpart/(.*)$ /$1 [R=301,L]
RewriteCond %{HTTP_HOST} ^www.somewebby.it$ [NC]
RewriteRule ^(.*)$ https://somewebby.it/$1 [R=301,L]
Results:
always having https and no www
hiding parts/anotherpart and showing https://somewebby.it/this.php
The problem:
The requested URL /this.php was not found on this server.

Answer 1

You can replace your current code by this one in your htaccess (which has to be in root folder)
RewriteEngine On
RewriteBase /
# if "www" or http -> redirect to https://domain.com/xxx
RewriteCond %{HTTP_HOST} ^www\. [NC,OR]
RewriteCond %{HTTPS} =off
RewriteRule ^(.*)$ https://somewebby.it/$1 [R=301,L]
# hide "parts/anotherpart/"
RewriteCond %{THE_REQUEST} \s/parts/anotherpart/([^\s]+)\s [NC]
RewriteRule ^ %1 [R=301,L]
# silently rewrite back to "parts/anotherpart/"
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{DOCUMENT_ROOT}/parts/anotherpart/$1 -f
RewriteRule ^(.+)$ parts/anotherpart/$1 [L,QSA]