Headings and paragraphs
Making sure your readers have no issues reading your text requires you to structure it. Especially larger pages should not be written as a massive blocks of text. Structure it with paragraphs and split those into nicely readable chunks with HTML headings in between. Ideally, the main heading at the top of the page should be an H1 (this is the <h1> element in your HTML), and subsequent headings H2. If you need more hierarchy, you could even use H3s but don't go deeper than that.
This structure is good for your readers but also helps Google to find structure and hierarchy in your pages. The most important thing to do is to make sure there is a logical structure in your headings. Ideally, start with an H1, but starting with an H2 will not kill your SEO rankings either. The next heading level should be either an H2 or H3, whatever comes next, similar to what you would normally do when you build a Word document.
In an ideal world, use an <h1> as the starting heading on every page. Always one and never more than one H1. Mostly you would want the title of the articles in Joomla to be the <h1> of the page. In Joomla 3, this used to be a problem, as by default, articles had an H2 heading, but in Joomla 4, H1 headings are created as desired, so there is no need for funny workarounds.
After you have arranged an H1 on every page and also some H2's if needed, make sure to keep structuring your text. Usually, I do not go deeper than H3. For my module titles, I often use the H3 heading. Use <p> tags for the body text.
Within your text, you can put emphasis on specific words by using the <strong> element. Google recognizes this and usually, your CSS will make it bold so users recognize it as standing out. Also, italic can be used for specific purposes. Use it sparsely though and only if it really makes sense. Don't just use it for SEO purposes...