Hi, try searching on Youtube for dieting for muscle gain; most reputable channel I can recommend is Dr. Mike from Renaissance Periodization (RP). "But I don't wanna look bulky or add muscle, just gain weight." That is the healthy outcome of weight gain. What's the alternative? Gaining fat. Plus, anyone worrying about looking too muscular is at no risk of gaining that much muscle.
To gain mass healthily (i.e. gain muscle w/o too much fat), the #1 requirement is to be at a calorie surplus (duh). To make sure that's not all fat, you must do resistance training. No getting around that either, sorry. It can be bodyweight, gymnastics, etc. but the tried & true, beginner friendly way is by lifting weights. So next tip is to either get a gym membership or start investing in a home gym. RP also has great content for beginners looking to gain muscle, but the best resource for a complete newbie IMO is the r/Fitness wiki. Perfect starting routines for beginners plus an overview of the basics.
The most important part about training is progressive overload i.e. lift more weight, reps, sets, etc. than last time. A good lifting program (like those from the wiki) get you there, but your body needs fuel to grow. Ideally, the extra calories come from more healthy food like whole grains, lean meats, fruits, veggies, etc. That's tip #3: eat more, responsibly.
Honestly, those 3 tips alone will probably get you to your goals, whatever they are. Everything else has like <10% impact at best. Just eat more, responsibly, while lifting heavy ass weights that get heavier over time, and any skinny guy (or girl) will fill their frame out appealingly without becoming a blob.