Build upper body strength: no weights or gym?

The best exercises for upper body strength are Bench Press, Overhead Press, Barbell Rows, Push-ups, and Chin-ups.

Honestly....if you want to build up a significant amount of strength(in order to look like a bodybuilder or fitness model)....joining a gym would be one of the best options.

Most gyms have the tools in order for you to achieve actual progressive overload(combined with a diet to support maintenance/growth).

If you don't want to do that, then buying weights(like a rack of dumbbells) would be the next best & cheaper option. You do not need to join a gym, work with machines, or even use barbells if you have a dumbbell rack along with weights....

With bodyweight exercises(such as pushups, pullups, and situps)....you'll build a decent amount of strength with them, but they're not optimal when it comes to achieving progressive overload. The exercise will get easier for you to do over time, and eventually it'll get to a point where your body really doesn't have a reason to grow bigger or stronger, because you're already used to doing it.

Sure. Push ups, dips, and various types of curls. As Jude points out, you can use water bottles, 1/2 gallon jugs, or Landry detergent bottles as hand weights.

https://www.youtube.com/watch?v=HRvFxrFGqA4

https://blog.mapmyrun.com/the-8-minute-no-equipmen...

Push ups, pull ups. Curl a gallon of water.