The truth is that you can lose fat without weight lifting.
You can lose muscle without weight lifting.
If you want to only do body weight exercises and stay out of the gym, you'll have to get crafty.
You don't need iron weights. But, you do need to increase the tension load your muscles are under if you want them to get larger. Also, you'll have to eat more calories than you burn to grow muscle.
If you just want to maintain your current size and get stronger, you'll have to force your muscles to do more work. High weight, very low reps, several sets. Like, 6-8 sets of 3 with 85% of your one lift max.
To gain muscle size, you'd typically need to do moderate weight, moderate reps. Like 4 sets of 8-10 with 65% of your one lift max.
To gain stamina/endurance in your muscles, 2 to 3 sets of 12-18 reps with about 50% of your one lift max.
Having said all that, the internet is absolutely filled with various conflicting details about how to do this or that. It can be really tough to sort out the details, as to who's telling it true, and who's just trying to sell supplements.