Phuuskon Thanks for posting your question in Microsoft Q&A. Logic App (Standard) supports both system-assigned and user-assigned managed identity and refer doc: Consumption versus Standard logic apps & Where you can use a managed identity for more info.
However, for Standard built-in connectors, only system-assigned managed identity is supported right now, and unfortunately, user-assigned managed identity is not supported. I understand this is a great feature to have and suggest you posting this feedback directly to our product team via Logic Apps feedback. Others with similar interest can upvote your idea too and helps our product team to prioritize as well. For managed connectors, you can check out this doc for adding user-assigned identity.
Also, I am passing this feedback to our doc team (#109372) to add this note to Authenticate with managed identities doc. I hope this helps with your questions and let me know if you have any other.
If you found the answer to your question helpful, please take a moment to mark it as "Yes" for others to benefit from your experience. Or simply add a comment tagging me and would be happy to answer your questions.