As brief as possible. As many different stakeholders will read the “solution strategy”, its explanation shouldn’t be overly technical.
State these decisions which gravely influence(d) your architecture. Refrain from overly detailed explanation, don’t describe possible alternatives or even implementation guidelines.
You can (or should) dive into such technical detail in arc42 section 8 (crosscutting concepts), where you can elaborate on the how and why of your approaches. If only single building blocks are concerned,even the building block view (arc42 section 5) might be the appropriate location for this information.
Provide hyperlinks or at least references to sections or documents giving additional information or detail.
It’s often a great idea to refer to specific quality requirements or goals when explaining solution approaches - see the examples in question C-4-3 (examples for solution strategy).
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