Just for the intellectual challenge, I'll suggest that's partially true.

In theory, it removes sufficient weight from the rough opening, that the opening is self-supported (for lack of a better description).

Depend on the height of the opening, that weight then moves diagonally back to the structure (I believe). In other words, a "header" on a 3rd floor of a structure removes sufficient weight in a RO, only to a certain "depth". That load eventually moves back to the fulcrum/base and a 2nd header would be required to put a window into the 1st floor of that wall.

Am I missing something?

& again, if we're talking precast concrete, it's a whole other can of worms.

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Your research word salad is impressive but you know we are talking about a window being removed and a door being installed in a single story wood framed home on a concrete slab. My research was done in 30 years building homes.

Amarillo is an older model name. Either a frame or block construction. Usually between the 6’s and north. Poured walls off site (tilt wall usually onsite) was the new construction south of 44, fully completed then dropped into place with a crane. More than Twenty years ago our family did onsite poured walls, for larger homes, with an occasional on site tilt wall, instead of block.

OP hasn’t stated the wall construction, guessing block. I do wonder why the window wasn’t changed at the same time me as the lanai was added.

No research required, the "word salad" is mine, based on building $100,000,000 worth of commercial buildings & at least 100 homes, back in my younger days.

I'm happy to explain it in other terms if you'd like?

(& the OP did not specify it was a "wood-frame" home and a subsequent poster specifically asked about the process if it was "tilt-up" construction.

I
Pretty obvious that with all your experience “Building “ that you have never been hands on doing any of it. Also , that specific Villages house plan is wood frame only

You're exactly right. Someone has to be there, to tell the "hands on guys" how to do it properly, which is why we have Architects, Structural Engineers and Building Inspectors.

In this case, everyone seems to be forgetting that changing a window to a door, triggers Level 2 Compliance with the IBC (which FL Building Code is derived from (2015 IBC). You can't just go changing doors to windows, willy, nilly ... at least not if you want the home to be Code compliant. Of course, most of the handyman types around The Villages don't have a clue and don't worry about what happens when someone goes to sell a home and they find out that Mr. Handyman didn't do a job right.

So unless for some ridiculous reason, FL decided not to adopt the relevant section of the IBC, replacing a window with a door, isn't a job for a handyman.

Fortunately the OP knows that, which is why he titled this thread, "Looking for an Architect ... ".