Doing a regular workout with weights in your youth (teens to late 30s) is going to cause long term chronic injuries in old age (50s or later). Is it a true statement?

I wouldn't worry about it. I started lifting for sports at age 14. I'm 56 and still workout hard for 45 mins. - hour daily. Variety is key. Too much of one type of exercise at too high of an intensity level can cause injury. When injury occurs (as it probably will at some point) use RICE and try to avoid ibuprofen, NSAIDs. Long term discipline in physical exercise is right up there with prayer/meditation in terms of quality of life decisions you can make.
Is it possible to gain endurance and lifting strength at the same time?

Definitely. Choose a rep range that compromises between maximum intensity (heavy weight) and volume. I'm using 'endurance' quite technically here, meaning muscle endurance. However, you may be intending something closer to stamina. So: to break it down: heavy weights at low reps = strength focus; moderate weights in a higher rep range diversify, developing some

Do squats help you develop abs?

Do squats help you develop abs?Yes, its named the king of all exercise. When squatting, you directly target the primary muscle; Quadriceps and Glutes. However, you also engage your core muscles - your Abs and Lower Back secondarily.While squatting, the amount of strength required from the core

Is it possible to get bigger muscles without any equipment?

Definitely!You can do all sorts of things to get size with just your own bodyweight. Focus a lot on time under tension. Also high reps are good. Your own weight should be enough. For example if you are a beginner master