Trying to include c++ library, but "fatal error: xtensor/xarray.hpp: No such file or directory"

Question

I'm a c++ beginner, I have a small amount of experience using python and numpy and wanted a matrix library that resembled it that I could use in c++ and I thought I'd try xtensor. I'm using ubuntu 20, and followed the install instructions on the github to install using the conda package (so conda install -c conda-forge xtensor). I created a file containing the very first example on the github, the basic usage example:

#include <iostream>
#include "xtensor/xarray.hpp"
#include "xtensor/xio.hpp"
#include "xtensor/xview.hpp"

xt::xarray<double> arr1
  {{1.0, 2.0, 3.0},
   {2.0, 5.0, 7.0},
   {2.0, 5.0, 7.0}};

xt::xarray<double> arr2
  {5.0, 6.0, 7.0};

xt::xarray<double> res = xt::view(arr1, 1) + arr2;

std::cout << res;

I navigate to the directory containing the file and run "g++ --verbose -o testXtensor testXtensor.cpp", but it immediately fails and the terminal returns:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.4.0-1ubuntu1~20.04.1' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-Av3uEd/gcc-9-9.4.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1) 
COLLECT_GCC_OPTIONS='-v' '-o' 'testXtensor' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/9/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE testXtensor.cpp -quiet -dumpbase testXtensor.cpp -mtune=generic -march=x86-64 -auxbase testXtensor -version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o /tmp/ccqFVVyC.s
GNU C++14 (Ubuntu 9.4.0-1ubuntu1~20.04.1) version 9.4.0 (x86_64-linux-gnu)
    compiled by GNU C version 9.4.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/9"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/9/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/9/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/9
 /usr/include/x86_64-linux-gnu/c++/9
 /usr/include/c++/9/backward
 /usr/lib/gcc/x86_64-linux-gnu/9/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++14 (Ubuntu 9.4.0-1ubuntu1~20.04.1) version 9.4.0 (x86_64-linux-gnu)
    compiled by GNU C version 9.4.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 65fe925b83d3956b533de4aaba7dace0
testXtensor.cpp:2:10: fatal error: xtensor/xarray.hpp: No such file or directory
    2 | #include "xtensor/xarray.hpp"
      |          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.

What have I done wrong? To the extent I understand the report in the terminal it seems g++ doesn't know where the xtensor library is and consequently I have nothing in my #include "..." search starts here: section, but I'm really not sure how to tell it how to find it and I'm confused why the instructions for this aren't included in the install instructions if this is something you need to do manually every time? My g++ version is 9.4.0 and conda is 4.14.0, if that matters. Any advice gratefully received!