Underneath “Tools” in the menu bar, there is the “Protect Sheet” option, which pops this sidebar on the right-hand side of the screen:
Which works well, however there doesn’t seem to be an option to protect all sheets in the spreadsheet.
After googling this, I found only one relevant thread:
Protect All Sheets by Default
Which is just a collection of complaints of various users who want the same feature.
Is there an option to protect all sheets in a Google Sheets spreadsheet?