Dead simple test for these methodologies.
"Do you plan fixed requirements, fixed time?"
"Yes."
"You are doing waterfall or you are doing nothing. It's 100% not agile or scrum and it's 100% not DSDM. You are denying the wealth of experience and knowledge over the past 70+ years on how bad software estimates are and how fast requirements change"
The main reality governing modern methodologies is that in today's world the requirements change faster than they can be developed.
This literally destroyed waterfall. Projects ended up in endless cycles of refinement to meet changing requirements and over ran by years and millions.
When the customer finally gets their solution, regardless of how well you did the UAT and requirements verification, they WILL have changes they will still want. So if you stop there, you leave the customer with only 60% of what they wanted and 40% of stuff they didn't.
Most of the methodologies stood up and manicured to address this went straight to those problems.
Most look at "iterative" models. Basically taking the "problem" from waterfall - too many costly re-cycles - and made it a feature - short rapid iterations of the process, time boxed so that a complete loss of an iteration is not a massive impact to the project overall.
Agile more specifically then addresses a few other problems from "waterfall".
"Ivory Towers". In waterfall there was a tendency that silo'ed "high level" "departments" and teams handle all the "clean" paper work in isolation. This paperwork is then handed to the engineers to do the lower level, 'down in the dirty' design. Eventually that output goes to teams of programmers and testers to develop. By that time the path back to "Knock on the wizards door" is guarded by the dogs of management. That or the person left the company or simply is too busy to answer clarifications. In large private or government sector projects there can be over a YEAR between the commencing meetings and the developers seeing the project for the first time.... after all the requirements are set in stone (really in sand) and the deadlines and deliverables have all been written in stone and blood.
Agile's approach to solving this is to incorporate a "role" for each step in that cycle in the "iteration", including the customer. Note: Not the customer's manager or business leader, but the person who will actually USE the solution.
"Who knows best?"
Agile also tries to address this. The answer, whether business or management agree is irrelevant, is "the end user and the developer". The end-user knows what they need, what works and the role/process/system in question well. The developer knows well what "can" be delivered with the current up to date tools, techniques, frameworks, platforms etc. The developer is also in his/her most ideal position to give an accurate estimate of "What can be delivered within the next iteration and shown directly to the customer first hand."
Develop -> Demo -> Discuss -> LOOP
There are no managers involved in this process. None. That is a specific part of the methodology. None should be needed and thus having any will slow things down. It is literally like having the restaurant manager getting in the face of the Chef and a diner while they are discussing the dish on the table.
Now, there has to be "oversight" of course. It's just that in Agile it is applied "out of band" from the dev stream. The interface to the dev team is via "roles" like "Product Owner", "Scrum master" (from scrum). (Should be noted that Agile is not a single monolithic methodology, instead is is a modular collection of tools, techniques and principles, most already available off-the-shelf of academic papers (like Scrum, Kan-ban, Lean et.al), , others "borrowed" and mutated, like DSDM. Not all are from software or even tech. "Lean" comes from production lines.
There is no point continuing, the point was made at the "Fixed time and requirements" boxing. This is the failure point for Agile as we know it. This is done in 90% of the teams I have been in that claim to be agile. They all have at VERY least fixed deadlines with fixed functionality contained at the "Epic" level. Totally failing to accept the academic research which clearly shows "Software estimation on the large scale is so bad, you might as well roll a dice." If you stand up a static plan, you stand up for failure. It's not even like it's 20% of projects it's 99%. 99% of software projects overrun time and budget and under deliver on functionality.
The other papers show that acceptance and embracing of this fact can actually result not only in better "estimation" and thus better target dates, but can significantly increase the amount of "use" and "quality" delivered to the customer.
Management do not study, nor are they required to study these kinds of materials. It's not their job. "We" do. The Software Engineering capability.
Today most companies see "Scrum master" or "Product owner", they see "Manager", they lift the "cookie cutter manager" job description and the practically BEG people to take it. ZERO requirements for understanding software engineering or it's methodologoes. It is assumed that "Projects are projects are projects" and "Managers are manager are managers". "Any manager can manage a complex software project, it's just management right?", "What do you do? Send a few emails? Fill out a few strawmen in Excel? Ask the team to produce a slide of your PP deck each? Easy right?"
It's literally like being cast back in time to the 1980s before we learnt how to engineer software. The quality and timely-ness of deliverables is also headed retrograde fast.
On the degradation of the role, "Software Engineer" I remind my employer that "I" bear the civil and legal responsibilities and ethics of my profession to the people and the state. Because of education, qualifications and so many years in the role, I can be held accountable for it in court. The British law considers me a "Qualified practising engineer" and so am held to that regard. I can be summoned as an expert witness and can receive charges of negligence that the company I work for cannot protect me from.
Managers don't. Because they are not "professionals" and have no voluntary, self-regulated, or chartered responsibilities or body of.