Hi,
that′s a problem. I′m a beginner and had this to. I spend like 1 Month reading (included “the 4 Esmi-Links”) but this is difficult to understand.
I just downloaded nearly all php-files and controlled them, comparing with a fresh downloaded WP and Theme files. (I was frightened to delete WordPress and install again, I was worry that it will not work again like it did).
In my case, there was a strange line at the bottom of EVERY index.php and the header.php. Efectivly, after finding the strange code-line (with numbers and the word “eval”) it cost me 10 minutes of work.
If you are not so lucky to find it, I recomend this service. I just was at the point to give up and found this https://sucuri.net/. And now I use this to prevent https://www.siteautobackup.com/index.php
I hope you can find the virus, good luck. This really sucks.