The first thing you need to know when filling out a PDF form, is whether it is an "interactive fillable", or a flat document.
If it is "fillable" then it will contain interactive form fields that can be typed into without selecting any tools. Just click into the field and type.
But, if it is a flat dumb form, i.e., it's just regular page text with no interactive fields, then use the "Fill and Sign" tools, which are meant for this purpose. I believe this is essentially the same as the old "typewriter" tool. Adobe likes to have fun confusing customers by renaming tools and moving them around to different categories.
Just as a general rule, never use the "Add Text" tool under the "Edit PDF" tools, unless you know what you are doing. This tool edits the page text, which is why it moves the existing text to the right. It can also introduce all kinds of errors into the PDF. It is intended to only be used by experts to make small changes in documents.
BTW: The old typewriter tool still exists, it's just hidden. Run this code from the console window to activate it.
app.execMenuItem("TypewriterMainToolBar")
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