If you follow the official instructions from here, you will end up downloading over 100 GB of system images. This is due to this line in the repo manifest:
<!-- Test infrastructure uses these -->
<project path="prebuilts/android-emulator-build/system-images" name="platform/prebuilts/android-emulator-build/system-images" revision="main" clone-depth="1" />Instead of emu-master-dev, select the emu-master-qemu branch to only pull dependencies for building the emulator.
If you are targeting aarch64-linux, cross-compile from an x86_64 machine, as there are no prebuild toolchains that run natively on this target. I recommend just using the latest Ubuntu LTS or Debian, as this is what the python script expects.
mkdir emu
cd emu
repo init -u https://android.googlesource.com/platform/manifest -b emu-master-qemu --depth=1
repo sync -cj `nproc`
cd external/qemu
pip install absl-py
pip install urlfetch
sudo apt-get install crossbuild-essential-arm64
python android/build/python/cmake.py --noqtwebengine --noshowprefixforinfo --target linux_aarch64