How do I share a file in Teams? 

You can share files in a one-on-one chat, a group chat, or a team channel. Select the paperclip icon below the box where you type a message to upload a copy of your original file. You can also upload a file by going to the Files tab of a chat and selecting Share.

Where are files stored?
Files you share in a channel are stored in your team's SharePoint folder, which is found in the Files tab at the top of each channel. Files you share in a private or group chat are stored in your OneDrive folder, which is found in the Files tab at the top of a chat. These files are only shared with the people in that conversation.

Can I share a file with someone outside of my team?
Only team members can see team files. You can share a file with people outside your team, if they are also using Teams, by sending it to them in a chat.

How do I co-edit files within Teams?  
Files uploaded and shared to a team are accessible to every member of the Team. In Office for the web, Office for Android or iOS, or the latest applications from Microsoft 365, you and your team members can co-edit Word, PowerPoint, or Excel documents, or comment on Visio files. You don’t need to check files out or worry about colleagues having the document open. Just open the file you want to edit, and if other team members are also editing it, your changes will be seamlessly merged as you work.