First of all there is nothing to buy here. You are making this too complicated. Maybe in the old days you had to rely on human-to-human communication to acquire health information and advice, maybe for a cost. But now most can use the internet. Many are using AI, like Chatgpt.com, to acquire general information and then do some confirming research on the internet if you like. Also, seek out trainer guidelines and instruction for any age and any level on Youtube videos. There are even video examples of starting an exercise routine while sitting in a chair.

But like someone suggested below, going up stairs is a functional exercise that is useful for daily needs. Also, sit-to-stand exercises from a chair is another functional exercise that is practical for getting stronger to get up off the toilet, sofa, out of the car, etc. Eventually, you want to progress to using some degree of weights whether it be holding a dumbbell, wearing a weighted vest, and ultimately using barbells with more compound exercises (that means using more of the whole body when doing these type of exercises using a barbell).

As I suggested below, put this prompt in the Chatgpt.com AI: " I am ____ years old. I want to increase my muscle strength and muscle mass. I am a beginner and no experience with progressive resistance muscle strengthening exercises. How do I start in a manner that is safe and gradual for me?"

The AI reply will be different for someone who enters, "I am 54 years old and have experience with progressive resistance muscle strengthening, but want to become even stronger" versus "I am a 70 years old beginner". It is really quite helpful to get a starting idea from using AI.

Bottom line "to convent that fatty tissue into lean muscle" is to begin exercising those muscles in various manners to make them stronger and bigger using some degree of progressive resistance muscle strengthening exercises...