These are really big questions, and I get a bit tired just thinking about how to tackle all of them, so I will pick the bits that I think are most interesting and talk about them. I think /u/khosikulu is probably more of an authority on this, so you should do a dance to summon him.

How did German colonialism affect Namibia? Too big of a question for me to manage, but this cannibalized version of an Africa Today article at least gives you a sense of what things are like there now.

Today's white Namibians are the descendants of the German and South African colonisers who ruled the country from 1844 to 1990. They make up 6% of the population, but, having held on to the privileges of apartheid, control 90% of the land. 40% of this land is commercial and fenced off, and considered private property. Some of its owners are absentee European landowners who live permanently in Italy, Germany and elsewhere.

And almost everyone else is black, incredibly poor, and lives in a shack in a slum. So there's that.

What did German colonialism look like while it existed? Not very different than anyone else's colonialism in many respects, really. Except for the concentration camps.

The scholarly source for this info is The Kaiser’s Holocaust: Germany’s Forgotten Genocide and the Colonial Roots of Nazism, by David Olusoga and Casper Erichsen (Faber & Faber, 2010). This is a really beautifully written book. Yes, I quibble with some of their statements, but really, in a whole big long book, it'd be weirder if I didn't find some questionable things. Olusoga and Erichsen argue, very very convincingly, that to understand the Holocaust, you have to look back to Namibia.

What Germany’s armies and civilian administrators did in Namibia is today a lost history, but the Nazis knew it well. When the Schutztruppe attempted to exterminate the Herero and Nama peoples of Namibia a century ago, Hitler was a schoolboy of 15. In 1904, he lived in a continent that was electrified by the stories of German heroism an African barbarism emanating from what was then German South-West Africa.

How could the two things be similar? The Germans wanted what the Herero and the Nama had--their land--and, when the Herero and the Nama fought back instead of just giving in, the Germans made war on their opponents, facing them on the field of battle but also imprisoning as many of the men, women, and children as they could. And not the kind of imprisonment where you get to go home after serving your term.

Until perhaps only 30 years ago, Luderitz’s oldest residents had their own memories of what happened here in the first years of the 20th century; they said nothing. Today it remains a secret. The tourist information office on Bismarck Strasse has nothing to say on the subject, none of the guidebooks to Namibia mention it and most of the history books they recommend as further reading are similarly mute. Yet what happened in Luderitz between 1905 and 1907 makes it one of the pivotal sites in the history of the 20th century.

The experiment took place on Shark Island, a squat mean-looking ridge of rock that lies just across the bay, in full view of the whole town. It was in its way a resounding success, bringing to life a new device: a military innovation that went on to become an emblem of the century and take more lives than the atom bomb. For here, on the southern edge of Africa, the death camp was invented.

80% of the Herero were exterminated, as well as at least 50% of the Nama people.

A note about personal biases/responses: I'm personally still wrestling with how I feel about some of the historiographic approaches that have been used for trying to understand what the Germans did in Namibia. As a Jew, I find it particularly troubling that what happened in Namibia can be so easily forgotten by the rest of the world. I find it troubling that more effort has not been made to connect what happened to my people in the mid 20th century with what happened to Herero and Nama people in the early 20th century. In Berlin today there is a monument to Holocaust victims--WWII era ones. But when I was there, I saw no mention of Namibia. I went to the Holocaust Museum in Berlin, too. I saw no mention of Namibia there. It was weird to be a Jew traveling alone in Germany, trying to find scraps of information about her family's history. I think it would only be weirder to be a person of Herero or Nama ancestry and to not see your people's history reflected in Germany at all, especially when there are definite, visible efforts made to remember the WWII Holocaust. At the same time, I question whether my personal and emotional feelings about the words "Germany" and "genocide" make me utterly unreliable to answer your questions. So you should keep that in mind when evaluating my replies.

edit: I fixed one of the sentences that got away from me. and left the rest.

edit2: I had no idea this video (7 mins 22 seconds) existed--it's a youtube snippet of a documentary and it's the bit about Shark Island. One of the authors of the book I mentioned is an interviewee, in the bit I watched.

Based on the tone of your question, I believe you are trying to get homework answered for you. Please prove the good-faith intent of your question by adding more detail to your inquiries and elaborating on exactly why you would like these points answered.