Are untrained muscles (and related tissues like ligaments and tendons) different from strong trained muscles, except for being less strong? Similarly, should “untrained” people train different in the beginning? What would such a different kind of training look like? How much would this be more about specific exercises than the muscles themselves?
It's really that simple. The more hard work you do, the more conditioned your musculature and supporting systems are to doing hard work.
People who have physical jobs have muscles that are trained to do their jobs. If that involves carrying heavy objects from one place to another, digging, working with concrete, etc., then the body and its connective tissues are already adapted to doing hard work. The demands of lifting weights have the same impact on the body as working a physically demanding job.
What should an untrained person do differently than a trained person?
The body can get acclimated to doing work pretty quickly. Over time you'll be able to really push yourself to get stronger, more muscular, or whatever your goals happen to be. There are a few recommendations that make a lot of sense for someone new to lifting weights:
The more work you put in, the more you'll have an idea of what goals you want to pursue. You'll also have a better foundation to pursue those goals. There's a reason 3 days a week works better than 5 when you are first starting out. It's also best to start out with programs that are designed for beginners. They are designed to help you build strength relatively quickly, but more importantly, they help build confidence as you get stronger. They are built knowing the limitations of the average beginner.
The only preparatory work that somebody might want to do before starting a beginner strength training program is gaining the necessary flexibility and awareness needed for maintaining form on some exercises.
Areas that in some people require a bit of preparatory work are:
There is no need to focus on these items unless form is suffering on particular lifts.
As long as you can complete with good form a few repetitions of the intended exercise, there is no extra preparatory work that is helpful.
About increasing effort slowly, that is advocated by all novice strength training programs. These programs have you starting with something as light as the empty bar. They generally have you add a little bit of weight every workout. Depending on the person and lift, this can be as much as 15 lbs per workout, or as small as 2.5 lbs per workout. This is about giving your body time to recover and to adapt to the previously applied stimulus.
Yes, some muscles during a movement can be weak. That is why during a novice strength training program, the starting weights are below what you're physically capable of. You shouldn't be pushing your boundaries on day one. As long as you increment by an appropriate amount every workout, you'll continue to progress within your body's ability to adapt.