You definitely need proper diet to put on weight, and lift heavy weights. I was always skinny pretty much all my life, and I was able to gain 5-10 pounds of lean mass during last year (mostly summer). Because of Covid, I had to move outdoors, and discovered calisthenics. That changed everything. I feel so much more motivated to work out now. During the summer I used to go to Stanley Park (Vancouver) and stayed there working out for hours listening music/podcast. And here is the thing: you need to find some sort of exercise that you like to do, or you will stop doing it. Regarding gaining lean muscle, the general advice is to reach muscle exhaustion when doing the reps. I found that to be the key, not so much rep count (even though I follow the traditional 6-12 rep count). Anyway, you should find a lot of advice on the web.

And of course, the diet, which has been the most challenging for me. I have a hard time eating a lot of healthy calories. It's just too much food :P! I read some recommendations on drinking a lot of milk, but I guess we all have have a limit of lactose we can handle [citation needed], and it did not go well.

Anyway, I am trying to make it into a habit/hobby, studying my body and my mind, trying to find what works for me. Hope it helps.