They are filtered from the library's Elementary collection at the difficulty tier that fits 5th grade. The wider school-level collections are linked under "By school level" on this page.
A clear question, a hypothesis written before testing in if… then… because form, one independent variable with the rest controlled, at least three trials at each setting, and a conclusion that follows the data.
A research question and hypothesis, the independent and dependent variables, the variables to hold constant, a full materials list with costs, numbered steps, safety guidance and a data table you can fill in as you test.
It varies, so every project page states a cost estimate and a realistic time estimate up front. You can rule a project in or out before buying anything.
Some do. Every project lists its hazards and whether adult supervision is required in an at-a-glance panel on its page, so you can check suitability before you start.