You mentioned that I have to lift heavy to increase muscle mass + bone density, but what does that mean and what if I’m not ready to lift heavy weights?
Without getting overly technical, you have to lift substantially more weight than what you might in daily life to stimulate bone or muscle growth or to preserve what you have.
However, how much weight is relative to your current level of strength and conditioning. We’re not going to have you lift a weight that you are not ready for. When we say “heavy” all we mean by that is that it feels physically challenging (but not painful) by the time you’ve hit 5 to 8 repetitions, depending on the exercise.
Beyond that, if an exercise is not appropriate for you, there are numerous ways to adapt it and modify it, so it makes sense for your current level of mobility and strength.
In short, it’s possible to challenge your muscles and your body without practicing exercises that make your joints feel terrible.