To make progress, you need to "surprise" your muscles by periodically changing exercises and increasing the weight. While varying exercises is straightforward, you can base your weight choices on how easily you complete the designated number of reps. If you're already struggling to complete the last rep, it's too soon to add more weight. On the other hand, if you can still do a few more reps with the usual load, it's time to add a bit more weight.
It may take some time to determine the right weight for each exercise, but eventually, you'll know exactly which weight suits you best. It should be heavy enough to feel a significant load for the last 2-3 reps of each set. However, you should perform those last 2-3 reps without violating the technique. For example, if you plan to complete 12 reps, you should feel like you want to end the set after the eighth repetition.
Remember, what matters most is not the weight your friend uses for an exercise or the weight I use. If you're a beginner, don't rush to increase weight, or you'll risk injury. If you've been working out for a while but can't achieve the desired results, go ahead and add more weight. You can only determine the correct weight for each exercise through trial and error. Be prepared; it may not happen right away. But eventually, you'll progress and achieve the desired results.