In my experience, Swahili wasn't widely spoken, really only in a region of East Africa around Uganda/Kenya/Tanzania - which I also realized many foreigners come to think of as "Africa" because that's where they go on safari. It was great to drive from Tanzania into Burundi, or Ethiopia into Djibouti and not see a single foreigner for a month.
Certainly French is the language of the West Coast and of many hundreds of millions of people. I'm guessing if it also had Nigeria's 200million it would be the dominant language on the continent.
English, again, is common in the places tourists frequent, but otherwise not widespread outside the capital cities.
The vast majority of people even in very, very remote mud hut communities spoke at least two languages. 3+ was very common.
People in Africa fluidly switch languages mid-conversation or even mid-sentence. In DRC for example, it's common to see a conversation in French and Swahili, or French and Lingala, or Lingala and Swahili. But most people will make an effort to stick to French when talking to a foreigner.
I've also heard Kenyans say that Congolese people speak Swahili weird because they use French loanwords.
For me, when I lived in Kenya English was the lingua franca for official/business activity. Some older professional Kenyans I knew learned Swahili only after learning English (and their mother language, Luo for instance). On the street, Swahili was standard.
The Tanzanians I met however, considered Nairobi Swahili not even 'real' Swahili and more of a mish-mash for disparate speakers who might otherwise choose their mother tongue when possible.
And then there's Sheng - a creole that is an even further evolution from Swahili and various other languages.