Good note by @brb@feddit.nl is what kind of license they use:
While our core solutions, the infrastructure protocol any-sync, and the data protocol any-block, are released as open source under the permissive MIT license, we distribute the remaining layers, including the middleware library any-heart, and applications like anytype-js, anytype-swift, and anytype-kotlin, under the Any Source Available License. This license grants individuals the freedom to review, modify, and utilize the code for personal, academic, scientific, research, and development purposes. However, for commercial use, consent from the Any Association is required.
This way, we guarantee everyone the right to use, modify, and distribute the data exchange protocol and the data format, ensuring that anyone is free to create any application on top of them. We guarantee free, non-commercial usage of the software and full transparency of the code. However, considering the substantial R&D resources required for the application layer, we believe that businesses and networks utilizing our software for commercial purposes should contribute towards its ongoing development, allowing maintainers to support and enhance the platform.
The Any Association, based in Zug, Switzerland, is an organization that will govern the rights to use the software and will provide an opportunity for other significant contributors to join a sort of digital cooperative and become the governors of the software as well. This empowers significant contributors to co-decide the next steps of product development and protects them from rivals’ abuse.
https://blog.anytype.io/our-open-philosophy/
https://github.com/anyproto/anytype-ts/blob/main/LICENSE.md
There is also another 'open source alternative to Notion' project called AppFlowy, which uses a AGPL-3.0 license. I will try to post about that project once I get more familiar with it