MISC resources regarding licensing.

SPDX Specs and REUSE

source: https://reuse.software/spec-3.3/

license identifier

SPDX-License-Identifier: GPL-3.0-or-later

copyright text (spdx format is recommended)

SPDX-FileCopyrightText: 2021 Jane Doe
SPDX-FileCopyrightText: 2019 Jane Doe <jane@example.com>
SPDX-FileCopyrightText: © 2019 John Doe <john@example.com>
SPDX-FileCopyrightText: Contributors to Example Project <https://project.example.com>
SPDX-FileCopyrightText: 2023 Alice Hack and (other) contributors to Project X <URL>

SPDX-SnippetCopyrightText: (C) Example Cooperative <info@coop.example.com>

compatible copyright text (not recommended)

Copyright 2016, 2018-2019 Joe Anybody
Copyright (c) Alice, some rights reserved
© Example Corporation <https://corp.example.com>

in case not for the whole file (in-line snippet comments)

# SPDX-SnippetBegin
# SPDX-SnippetCopyrightText: 2022 Jane Doe <jane@example.com>
# SPDX-License-Identifier: MIT

print("Hello, world!")

# SPDX-SnippetEnd

Reference

List of SPDX License Identifiers
https://spdx.org/licenses/

Re-using free software via writefreesoftware.org
https://writefreesoftware.org/learn/participate/derived-works/

Chossing a license via writefreesoftware.org
https://writefreesoftware.org/learn/participate/choose-a-license/

REUSE Specifications
https://reuse.software/

Intra-GPL compatibility matrix via gnu.org
https://www.gnu.org/licenses/gpl-faq.html#AllCompatibility