Understanding Makefile. make cannot link armadillo library

I am new to C++ and I am having trouble understanding how Makefiles do their thing with the g++ compiler.

I have successfully installed armadillo library (via apt) and have a very simple c++ program test.cpp, like the one below:

#include <iostream>
#include <armadillo>

using namespace std;

int main()
{
   arma::mat A;

   A << -1 << 2 << arma::endr
       << 3 << 5;

   cout << A << endl;

   arma::fmat B;

   B.randu(4,5);

   cout << B;
   return 0;
}

This works just fine if I compile manually like this:

g++ src/test.cpp -std=c++11 -Wall -o test -DARMA_DONT_USE_WRAPPER -lopenblas -llapack

I can manually run the program and it delivers the matrices as expected.

On the other hand, I have the Makefile template from the VSCode C/C++ Extension, which I have modifed slightly for including the LAPACK an BLAS Fortran libraries:

########################################################################
####################### Makefile Template ##############################
########################################################################

# Compiler settings - Can be customized.
CC = g++
CXXFLAGS = -std=c++11 -Wall
LDFLAGS = -DARMA_DONT_USE_WRAPPER -lopenblas -llapack

# Makefile settings - Can be customized.
APPNAME = test
EXT = .cpp
SRCDIR = src
OBJDIR = obj

############## Do not change anything from here downwards! #############
SRC = $(wildcard $(SRCDIR)/*$(EXT))
OBJ = $(SRC:$(SRCDIR)/%$(EXT)=$(OBJDIR)/%.o)
DEP = $(OBJ:$(OBJDIR)/%.o=%.d)
# UNIX-based OS variables & settings
RM = rm
DELOBJ = $(OBJ)
# Windows OS variables & settings
DEL = del
EXE = .exe
WDELOBJ = $(SRC:$(SRCDIR)/%$(EXT)=$(OBJDIR)\\%.o)

########################################################################
####################### Targets beginning here #########################
########################################################################

all: $(APPNAME)

# Builds the app
$(APPNAME): $(OBJ)
    $(CC) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)

# Creates the dependecy rules
%.d: $(SRCDIR)/%$(EXT)
    @$(CPP) $(CFLAGS) $< -MM -MT $(@:%.d=$(OBJDIR)/%.o) >$@

# Includes all .h files
-include $(DEP)

# Building rule for .o files and its .c/.cpp in combination with all .h
$(OBJDIR)/%.o: $(SRCDIR)/%$(EXT)
    $(CC) $(CXXFLAGS) -o $@ -c $<

################### Cleaning rules for Unix-based OS ###################
# Cleans complete project
.PHONY: clean
clean:
    $(RM) $(DELOBJ) $(DEP) $(APPNAME)

# Cleans only all files with the extension .d
.PHONY: cleandep
cleandep:
    $(RM) $(DEP)

#################### Cleaning rules for Windows OS #####################
# Cleans complete project
.PHONY: cleanw
cleanw:
    $(DEL) $(WDELOBJ) $(DEP) $(APPNAME)$(EXE)

# Cleans only all files with the extension .d
.PHONY: cleandepw
cleandepw:
    $(DEL) $(DEP)

I have passed the needed libraries under LDFLAGS = -DARMA_DONT_USE_WRAPPER -lopenblas -llapack. Nevertheless, this solution does not work. It looks to me like the compiler is unable to find the armadillo library, so I must have linked it somehow wrongly. It delivers:

g++ -std=c++11 -Wall -o test obj/test.o -DARMA_DONT_USE_WRAPPER -lopenblas -llapack
/usr/bin/ld: obj/test.o: in function `TLS wrapper function for arma::arma_rng_cxx11_instance':
test.cpp:(.text._ZTWN4arma23arma_rng_cxx11_instanceE[_ZTWN4arma23arma_rng_cxx11_instanceE]+0x25): undefined reference to `arma::arma_rng_cxx11_instance'
collect2: error: ld returned 1 exit status
make: *** [Makefile:36: test] Error 1

So, aside from the obvious question (Why does this not work?), I would as well appreciate if someone could help me clarify as well the following aspects:

On the one hand, rom the message error it seems that the command run g++ -std=c++11 -Wall -o test obj/test.o -DARMA_DONT_USE_WRAPPER -lopenblas -llapack does not include the name of the cpp file I wrote (as opposed to in my manual compilation, in which it works). Nevertheless, if I do not use armadillo, the Makefile recipe above works just fine. I see the Makefile somehow looking for all cpp files in the source code folder SRC = $(wildcard $(SRCDIR)/*$(EXT)), but I cannot see where is this forwarded to the compiler. Can someone help me with that?

The other thing is that, in my manual compilation, it seems to make no difference to pass the LAPACK and BLAS libraries as CXXFLAGS or LDFLAGS, meaning both of the following commands:

g++ src/test.cpp -std=c++11 -Wall -DARMA_DONT_USE_WRAPPER -lopenblas -llapack -o test

and

g++ src/test.cpp -std=c++11 -Wall -o test -DARMA_DONT_USE_WRAPPER -lopenblas -llapack

work just fine. As far as I have been able to read, I understood the flags before -o are meant for the compiler, and those after are meant for the "linker" (whatever that is). Can someone explain me what are the main differences between the CXXFLAGS and LDFLAGS? Why both combinations work? And what is the linker?

Thank you very much for your help.

Best,

D.