Confused: US/traditional notation issue - one octave shift??

I am new to MuseScore and am kind of enthusiastic.
Everyone can now create, orchestrate and play his music,
rehearse vocals with as many other voices and instruments
as he wishes, at any tempo, etc. etc. etc.
A complete composer's toolbox inside your computer.
However, what surprised me when I played and read
scores of a cappella groups like Pentatonix was that
they played correctly - but music was written one octave
below the real pitch, as I knew it from my studies of music.
I checked past discussions - and there was some issue
over selecting C3 or C4 as the "middle C" - that may be it
or not, but It looks like another long-time convention was
violated.
What I discovered after leafing through some standard American music theory books was that americans tie treble clef stave and bass clef stave at C4 not at C3 !!!!!!!!!!!
Or, in other words, the bass clef colon surrounds not F2, but F3 -
according to this notation, while the G (or treble) clef curls
around G4, not G3
Which means all scores in MuseScore must be written one octave lower to sound correctly -- compared to the decades of years (if not a hundred or so) of standard musical notation, which I learned as a child.
I.e. it looks like I was totally wrong for the whole 5 years of my
piano playing - using sheet music published all over Europe -
if we believe this newfangled notation convention, where the C on the additional line below the treble stave and the same C one ledger line above the bass key stave is not C3, but C4.
Now, this I consider an impossibility - and please tell me whether I am just confused over some trivial misunderstanding, or whether indeed US notation system (or some new standard or whatever else it could be) indeed shifted the scores one octave up - and now require music to be written one octave below the universally accepted past convention.
My chest voice goes down to E2/D2, which gives a pretty good point to
compare - while the Pentatonix score I reviewed placed the E2 sound in
the bass voice in its SATB transcription on one additional line below
the base stave, rather than immediately below the line surrounded by the
colon of the bass key, between lines 3 and 4 inside the stave.
So what is it?
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// if I am wrong in some trivial way, I will delete this message not to
pollute the forum after receiving an explanation. If this indeed is a real issue,
I will let it stay//