The great explanation by Bill Dunkan on so-called “waterfall” projects:

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Bill Duncan

Project management consultant and trainer. Primary author of the original (1996) PMBoK Guide.

Probably the MOST important thing to keep in mind is that there is no such thing as a “waterfall” project. Waterfall was defined by Royce in a 1970 article as a way to NOT do software development. Now, if you understand project management (which this AI bot clearly does not), you would know that there is such a thing as stage gate (also called phase gate) which is often called “waterfall.” But in phase gate revisions are NOT “costly and time-consuming” unless you leave them to the end. And that’s true for ALL projects. And the approach is generally “linear and sequential” but overlaps are both allowed and encouraged”