African flags carry a lot of design decisions made under pressure, most of them at independence. Algeria, Angola and Botswana each solved the same problem differently: what do you put on a flag when the point is that you are no longer somebody else's territory?
Why the designs changed at independence
Vexillology, the study of flags, is useful in post-colonial settings for exactly this reason. A great many African nations redesigned their flags after independence, and the redesign was the announcement. Dropping the colonial emblem was as much of the message as whatever replaced it.
Algeria
The Algerian flag came out of the fight to end French rule. The crescent and star mark Islam, which is central to Algerian identity, and the green and white stand for peace and purity.
The Flag of Algeria
View Flag →Angola
Angola's flag is more explicit about the war it came from. Red is for the blood spilled in the fighting for freedom, black for the African continent. The yellow half cogwheel and machete stand for industrial workers and peasant farmers, borrowed from socialist iconography.
The Flag of Angola
View Flag →Botswana
Botswana's flag is the simplest of the three and the least martial. The blue field is water, named on the flag as a vital resource rather than as scenery. The black and white stripes stand for harmony between ethnic communities and take their pattern from the zebra, the national animal.
The Flag of Botswana
View Flag →What the three have in common, and where they diverge
Liberation and unity run through all of them, but the designs are not interchangeable. Each country had to decide how far to move from colonial visual conventions, and they landed in different places. Angola went for tools and weapons, Botswana for an animal and a resource, Algeria for a religious symbol.
That work is not finished. New states appear, existing ones re-examine symbols they inherited, and political change tends to bring flag proposals with it. Contemporary design practice and public consultation have both started to feature in how those proposals get made, which is a different process from a liberation movement picking colors.