Widening the Search-ability of a Journal's Articles
Google scholar

You can find the general indexing policy of Google scholar here: https://scholar.google.com/intl/en/scholar/publishers.html#policies

And the more technical information is available here: https://scholar.google.com/intl/en/scholar/inclusion.html

In case of link rot, the articles should be findable via search keywords such as "google scholar indexing".

The non-technical summary is that the abstract or the first page of the articles should be vievable for everyone. Further, I think the article should look like scientific articles, with titles, possibly abstracts, text, references, and so on. The exact parameters are almost certainly unknown. I know at least one journal published as a blog, and it is not indexed by Scholar, while some websites are.

General visibility

Publish good articles, get famous editors, perhaps appear in conferences or sponsor them, if relevant for your field. Avoid being a predatory publisher or looking like one.

Maybe also look into the basics of search engine optimization (not the arcane stuff about how to cheat Google, but the basics like having metadata and website headers right) and usability of websites.