
Based on your description, I suggest you create a team site to share and collaborate between users.
In a team site, you could create a document library to store files. To share files with users outside of the team site including guests, you should enable external sharing at both tenant and site level.
References:
https://learn.microsoft.com/en-us/sharepoint/external-sharing-overview
https://learn.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off
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