The apt sources appear to point at the NVIDIA repository:
$ cat /etc/apt/sources.list.d/nvidia-l4t-apt-source.list
# SPDX-FileCopyrightText: Copyright (c) 2019-2021 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: LicenseRef-NvidiaProprietary
#
# NVIDIA CORPORATION, its affiliates and licensors retain all intellectual
# property and proprietary rights in and to this material, related
# documentation and any modifications thereto. Any use, reproduction,
# disclosure or distribution of this material and related documentation
# without an express license agreement from NVIDIA CORPORATION or
# its affiliates is strictly prohibited.
deb https://repo.download.nvidia.com/jetson/common r34.0 main
deb https://repo.download.nvidia.com/jetson/t234 r34.0 main