Hi Craig,
I need some help because I cannot handle my problem.
So, I installed Magento 2.3 as you did. Everything was ok, I set permisson, etc.
I did not get errors.
After I installed a theme, also was everything ok under installation process, but after installation a lot of directories and files permission were changed.
I made a photos before theme installation and after theme installation.
I see the new template well, but when I login to Magento backend and I would like to choose an other layout from this theme I get an error about permission problem.
Warning: mkdir(): Permission denied in /var/www/html/app/code/MageBig/MbFrame/Controller/Adminhtml/Theme/Save.php on line 265
I do not know how to set again that permission rigths what was before I installed the theme.
I used this command as a superuser: sudo chown -R magento:www-data /var/www/html/
and after I run sudo chown magento:www-data /var/www/html/
After I login as magento user and I used this command: find var generated vendor pub/static pub/media app/etc -type f -exec chmod g+w {} + && find var generated vendor pub/static pub/media app/etc -type d -exec chmod g+ws {} + && chown -R :www-data . && chmod u+x bin/magento
I am confused so much, I tired a lot of varians, but my problem is did not dissapeared.
One thing:
When I installed Magento 2.2.6 as your video instruction and after I used this commands always when I installed anything and works, but this commands not work now.
find . -type f -exec chmod 644 {} ;
find . -type d -exec chmod 755 {} ;
find ./var -type d -exec chmod 777 {} ;
find ./pub/media -type d -exec chmod 777 {} ;
find ./pub/static -type d -exec chmod 777 {} ;
chmod 777 ./app/etc
chmod 644 ./app/etc/*.xml