What do you mean the Teams meeting for external training? Does it have any difference from the normal Teams meeting?
From my experience, it has no problem to preload files, videos from Stream, whiteboards and polls in the regular Teams meeting. After schedule the meeting, then enter into the edit page, you can upload the resources in advance as below:
If I misunderstand your words, please give us more detailed information to know your situation clearly. Thanks for your understanding.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.