Thanks for this blog, which I enjoyed to read. I have a few comments, if I may, which should clarify a few points. Do not hesitate to come back to me in case you would disagree of have questions.
We understand what a black hole is. A black hole is basically created from dead stars.
Strictly speaking, there is also what we call primordial black holes, that consist of another category of black holes. They are however hypothetical and have not been observed. See here and references therein.
Does black holes can be seen? Light cannot come out of the black hole. As a result, black holes cannot be seen through telescopes.
I would like to come back to the above statement, that is not entirely correct. Somewhat, we have evidence of their existence through gravitational waves. That's one way to see them, as described for instance here.
As another possibility, we can use the shadow of the black hole as a mean to see them with telescope. This is what the Event Horizon Telescope did (see here).
A black hole has always swallowed the planets and various cosmic objects near it. These black holes can swallow stars bigger than our sun in an instant. Black holes can destroy our planet in a matter of seconds.
That is not always the case. This depends on the black hole mass. There are two competing phenomena inside black holes: matter accretion and Hawking radiation. Depending on which one dominates, the black hole will either grow, or evaporate. Doing the math, small back holes evaporate whereas heavier ones grow forever.
Cheers!