Problem is when you view page source and search for H1 tag, there are none. Since this is the title of the page and for SEO purposes, how would you customize the particle to create <h1> tags for the title?
The infolist particle does not use H1 deliberately (in fact it uses just DIV). The reason for this is that you should only have one H1 on a page for SEO purposes and the H1 tag is associated with the "page title" field of the menu item - as rendered by Joomla in component output). So, I don't recommend that you change this but if you really want to then you will need to create a particle override and alter the TWIG code but this would mean for any infolist having more that one item in it that each would have an H1 tag and you would therefore have more than one H1 on a page.
Please search forums before posting. Please make sure your post includes the version of the CMS you are using and a link to the problem. Annotations on screenshots can also be helpful to explain problems/goals. Please use the "secure" tab for confidential information.