basic question about wordpress HTML and formatting,[p] breaks…

Hi folks – I am new to hacking WordPress and this forum, so apologies if this is misfiled.

I am hacking a template I found somewhere and am trying to figure out where the basic HTML elements are set. Specifically, when I have a line return (between paragraphs or at the end of a line) sometimes the system sets a <p> tag, sometimes a
tag.

My current problem relates to embedding an Advertising Manager call, which somehow causes a <br /> after its use, instead of the <p> tag which starts the next paragraph. This clashes with the CSS style sheet because that paragraph is not embedded with a <p> tag, so its style (font size and face) are different.

Can anyone point me to the mechanism where these things are set? Or some kind of primer where I can get up to speed? Thank you for your patience if this is answered elsewhere or off-topic….