I've lost weight...ㅠㅠ

I'm a bit slim... well... just my belly sticking out???

I'm in my early 40s... I think I've lost about 1 to 2 kilograms like this.

The weight is almost the same...

Walking ten thousand steps every day.. climbing stairs.. sometimes doing home workouts..

I wasn't thinking much, but someone I know said it might be losing muscle mass... I'm scared...ㅠㅠ

I don't have much muscle mass...

I tend to eat a lot more compared to being thin... ㅠㅠ

What is the easiest exercise to increase muscle mass???

Walking 10,000 steps... When I get home, should I climb 20 floors of stairs and then take the elevator down...ㅠㅠ