Is it a new cancer or one spread from the previous?

[jianchun_zhu]

I was diagnosed with renal cell carcinoma in Mar. 2020. My left kidney was cut out. The doctor said that the cancer did not spread out. In Sep. 2020 the CT found that I was healthy, i.e. no more cancer was found in my body. But the in Dec. 2020 the CT found cancers in my left lung, one about 1cm, another about 1mm, and others of size that the doctor didn't mention. The doctor regard them as spreaded from the kidney cell cancer.
I received medicine(chemical) cure from then on. In the first year the doctor used  Inlyta+ Keytruda. But the cancer didn't shrink. grew a little bit in the year end. The second doctor used Sunitinib Malate for 1+ year, but the cancer didn't shrink either, but grew to 4cm. The doctor is planning to use another medicine that cures renal cell carcinoma and said he'll soon run out of treatment method.
My question is, how is the doctor sure that my lung cancer is spreaded from with my renal cell carcinoma?
(1) The first doctor said that she is also not sure the lung cancer came from my kidney. Only from the distribution(i.e. it distributed in a couple of places rather than one spot).
(2) I am in my 50s and my immune system works well basically, how came all the medicine had no effect on me? So naturally I think that it's because medicine only cures renal cell carcinoma has no effect on other cancers. I think probably it's new cancer rather that spreaded from renal cell.
(3) 3+ month after the kidney-cut-off surgery I move to a cheap dirty room to save money. Maybe it is the reason of my lung cancer.
(4) It's weird that no spread was found 4 months after the kidney-cut-off surgery, but spread was found after 7 months.
(5) The hostipal made mistake before in important treatment.
So how much do you think the probability is that my cancer was spread from previous instead of newly-happened? 100%? 90%? 50%?
Thanks a lot.

[specialmom]

That is a lot to take in. First, I"m sorry you are going through this. So sorry. You are probably tired and worried. Hang in there. I'm not sure where you are at in the world since people post from around the globe. Remember that there are also clinical trials that you can enroll in as the 'normal' course of medication dwindles. Did they ever biopsy the lung cancer? Hospitals make mistakes for sure. Cancer is also super sneaky. So, it's hard to say what happened here. I'm just sorry and hope the next treatment they give you does the trick