@imayid2 @aygart this conversation got broken into too many fragments. Here is the whole MB with my comments. I didn't include the second half of SK 26 where he goes on bashing those who put the strings in their pants. This is the simple reading of the Mishnah Berurah, even if you wouldn't have read it that way without knowing what the MGA he quotes is referencing.
(כה) על בגדיו
.ובכתבים איתא דטלית קטן תחת בגדיו.
וכתב המגן אברהם, דעל כל פנים צריך שיהיו הציצית מבחוץ
If you look at the MGA, he says "like the gemara (in menachos) says "ain tzitzis elah yotzei"". That gemara is discussing how long the strings need to be, and uses this braisa to say there is no minimum length because it just needs to stick off the edge like a fringe.
ולא כאינך שתוחבין אותן בהכנפות.
The MGA is extending it to say that if the strings are tucked into the corner of the beged it isn't "yotzei"
אך האנשים שהולכים בין העכו"ם יוצאין בזה
As the MGA concludes based on the Tosefos in Berachos
ומכל מקום בשעת הברכה יהיו מגולין [כדי הילוך] ארבע אמות.
This is a continuation of the quote from the MGA, who is clearly only referring to the strings tucked into the corner of the beged.
(כו) יראהו
כמה דכתיב: "וראיתם אותו וזכרתם" וגו'.
This is why the Mechaber says you should wear the whole thing out.
ואותן האנשים המשימין הציצית בהמכנסים שלהם, לא די שמעלימין עיניהם ממאי דכתיב: "וראיתם אותו וזכרתם" וגו', עוד מבזין הן את מצוַת השם יתברך, ועתידין הן ליתן את הדין על זה
Here he decries putting the strings in pants, but if he was talking about those who wear the beged under the shirt, the next sentence makes no sense.
ומה שאומרין שהולכים בין הנוכרים, לזה היה די שישימו הציצית בתוך הכנף.
Instead of tucking it into their pants they would be better off tucking it into the corners.
Please explain how when wearing the beged under the shirt, there is something to gain by putting the strings in the corners instead of just leaving them inside the shirt or pants?