Software
Why is my iPhone not showing software updates.
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Depending on your iPhone model, the built in hardware may not support any further iOS updates.
Update your iPhone, iPad, or iPod touch - Apple Support
For example, any iPhone prior to the iPhone 6s can't be updated to iOS 15.
Check these articles about it:
iOS 15 and iPadOS 15 are compatible with these devices - Apple Support
iOS 14 and iPadOS are compatible with these devices - Apple Support
iOS 13 and iPadOS are compatible with these devices - Apple Support