Worksheet protection helps a shared model survive accidental edits. Cells start locked, but that setting has no effect until you protect the sheet. First identify the cells colleagues should edit. Clear Locked for those input ranges, then choose Review > Protect Sheet.
The protection dialog lists allowed actions, such as selecting cells, formatting rows and using an existing AutoFilter. Give users only the actions the task requires. Making input cells editable doesn't require you to make the formulas editable that depend on them.
Protect Workbook is a separate setting. Structure protection prevents changes to the arrangement of worksheets, including adding, deleting and renaming sheets. A password can control who removes protection, but sheet protection doesn't encrypt the file or keep confidential data secret. Check both the input path and the protected formula cells before sharing a model.