First of all, let's make a small clarification by making the difference between free audit and paid audit. Or rather between automatic audit and manual audit. Some websites, like Woorank or Yakaferci offer you free audits (with some paid functions) of your site.
You enter the address of your website and abracadbra , you know everything that is wrong. c level contact list
In reality, these online softwares automatically audit your site. It checks a series of points one after the other. For example:
Meta tags
Title tags
page length
But also some more technical points:
url canonicalization
html code compliance
Woorank, for example, offers nearly 70 criteria!
All these criteria can be automatically checked by a computer, which then tells you what needs to be optimized.
The software behind the site actually uses an algorithm to check all the points planned in advance one by one, according to a series of very precise, and therefore very restrictive, criteria.
This type of audit is a good starting point to know where you are. But as is often the case, human intervention brings much more!