Is it okay to copy somebody else's words and sentences?
As I'm sure you know it isn't correct to just copy things from others. However, there are a couple of additional considerations. 

Something is really plagiarism only if it is copied without citation. Then, the author is claiming the work of others as their own. The safest way to avoid this is to use quotes, as you suggest, but certainly to cite. 

However, if some of the work you find is old, it is also possible that the standards have changed a bit over time and are now a bit more rigid. Citation and being clear about who originated ideas is the important thing, not, specifically, the punctuation. 

But another consideration, especially in fields such as mathematics is that there may be only one way to write some given idea in words. So, in a calculus textbook, for example, the definition of the derivative is nearly identical to every other calculus text on the planet. This sort of thing has actually been discussed in courts, as I understand it. 

But for your own work, be as clear as you can be. Cite anything you use and clearly mark what is being quoted using some reasonable typographical convention. Of course you also need to worry about copyright infringement, but that is a different issue. The sloppiness of others shouldn't bleed over into your own work. 

The question of copying without understanding is a deeper problem. I won't comment on that.