How to change make command

Question

In my ~/.config/nvim/ftplugin/tex.vim I have the following:

set makeprg=pdflatex\ -halt-on-error\ -file-line-error\ -synctex=1\ %
set errorformat=%f:%l:\ %m

however when I run make the command that I see is different:

:!pdflatex -halt-on-error -file-line-error -synctex=1 test.tex 2>&1| tee /var/folders/gq/vqbg5g255cqf7x_yn8v3tlyc0000gn/T/nvim9VTy4P/7

why is stderr redirected to stdin, what is that second command with tee doing, and how do I remove both of them?

I also get this at the end of the compilation (if there were no errors)

(1 of 16): This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex)

and if there's an error I get for example

(12 of 16): Undefined control sequence.

what does this last line mean, and how can I remove it?

Edit:

Output of pdflatex -halt-on-error -file-line-error -synctex=1 test.tex as called from a shell

$ pdflatex -halt-on-error -file-line-error -synctex=1 test.tex
This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2020-02-02> patch level 5
L3 programming layer <2020-03-06>
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/article.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-pdfmode.def)
(./test.aux) [1{/usr/local/texlive/2020/texmf-var/fonts/map/pdftex/updmap/pdfte
x.map}] (./test.aux) )</usr/local/texlive/2020/texmf-dist/fonts/type1/public/am
sfonts/cm/cmr10.pfb>
Output written on test.pdf (1 page, 16718 bytes).
SyncTeX written on test.synctex.gz.
Transcript written on test.log.

Output of :make with set makeprg=pdflatex\ -halt-on-error\ -file-line-error\ -synctex=1\ %

:!pdflatex -halt-on-error -file-line-error -synctex=1 test.tex  2>&1| tee /var/folders/gq/vqbg5g255cqf7x_yn8v3tlyc0000gn/T/nvim4TX4tu/4
This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2020-02-02> patch level 5
L3 programming layer <2020-03-06>
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/article.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-pdfmode.def)
(./test.aux) [1{/usr/local/texlive/2020/texmf-var/fonts/map/pdftex/updmap/pdfte
x.map}] (./test.aux) )</usr/local/texlive/2020/texmf-dist/fonts/type1/public/am
sfonts/cm/cmr10.pfb>
Output written on test.pdf (1 page, 16718 bytes).
SyncTeX written on test.synctex.gz.
Transcript written on test.log.

(1 of 16): This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex)
Press ENTER or type command to continue

Answer 1

why is stderr redirected to stdin,

In order to have both end up in the quickfix list.

what is that second command with tee doing,

man tee. It redirects stdin into a file and into stdout. It's an extremely useful command to log and watch the execution of commands that send a massive amount of lines into stdout and/or stderr.

and how do I remove both of them?

You don't want to. That's what vim :make is expecting and needs to in order to populate the quickfix list. See :h 'shellpipe'.

(12 of 16): Undefined control sequence.

what does this last line mean, and how can I remove it?

It means there is an error in your LaTeX file and that you should scroll back to find the exact error line number, and ask in a LaTeX channel.