The plugin "jooa11y" is part of the core since Joomla 4.1.0 and it is activated by default in the backend.
In the plugin it can be defined which area of the site is checked (default is the main area) and rules can be included or excluded.
The "Accessibilty Check" button appears in the toolbar (in addition to save, close, etc.) of already saved posts. If you click on it, a preview of the page opens, where the violations are displayed. In addition, some rules can be enabled or disabled here.
As with all automatic test systems, this is just one tool. It can only find violations of rules that are defined in the plugin. It can also report "errors" intended in the context of an article (e.g. no alt text on decorative images).