You could build qzxing as a library and then link this library to your project using target_link_libraries.

This was my complete misunderstanding of cmake and how it works.

# with qzxing/src copied to my project as QZXing
add_subdirectory(./QZXing/)
target_link_libraries(${PROJECT_NAME} qzxing)

My misunderstanding was from two things:

1. I thought the command add_subdirectory() was the only thing that should have been needed
2. That I had to use the library name that is specified in the QZXing/CMakeLists.txt, which, from this command add_library(qzxing "" ${SOURCES}) is qzxing.

The QZXing repo does have a CMakeLists.txt file, https://github.com/ftylitak/qzxing/blob/master/src/CMakeLists.txt, in its src dir. Which is why I thought I'd just be able to add the repo as a subdirectory to my main project, then use some cmake function, such as add_subdirectory(), to reference the .h and .cpp files.

TEMPLATE = lib

It is already built as library.
To make it properly with QMake you should create a https://wiki.qt.io/SUBDIRS_-_handling_dependencies project. You then add qzxing as one subproject and your actualy project as another one. Then in your project you add qzxing as dependency (see "Defining Project Dependencies: .depends attributes" chapter in the link above).

Does building as a library in Qt mean opening the QZXing.pro file in Qt 6.4 and clicking build? When I do so, I get the following error:

:-1: error: No rule to make target 'install'.  Stop.

What file is this install target supposed to be in?
This is the .pro file:

CONFIG += \
          enable_decoder_1d_barcodes \
          enable_decoder_qr_code \
          enable_decoder_data_matrix \
          enable_decoder_aztec \
          enable_decoder_pdf17 \
          enable_encoder_qr_code \
          #staticlib \
          #qzxing_qml \
          #qzxing_multimedia \

VERSION = 3.3

TARGET = QZXing
TEMPLATE = lib

include(QZXing-components.pri)

DEFINES -= DISABLE_LIBRARY_FEATURES

The only thing QZXing mentions about building as a library is https://github.com/ftylitak/qzxing#compile-the-project-as-an-external-library.