For the free option, is that 100MB for the available upload space?

In addition, for that available space when we download the files will that free up the space again for other downloads?

Yes, that is correct. Free account has a limit of 100MB upload space. To clear the upload space, you need to download the file and delete the submission with the attachment file. Keep in mind that deleted submissions is not recoverable. 

Here's a guide: How-to-Delete-Form-Submission-Data

Related guides:
How-to-Create-File-Upload-Forms
How-to-Change-the-Allowed-File-Types-Extensions-in-the-Upload-Field
Changing-the-File-Upload-Size-Limit

Hope that helps. If you need further assistance, please contact us again.