when you say God, in your mind you know who you're talking about (Allah)l
but when you say god, you talk about the word itself, not someone specific

for example
for some religions, when we talk about them, we say about their gods (god)l
but for them, they God as they have a specific god in their minds

and the opposite for us
they say god to ours
and we say God to ours

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but we, Muslims, use the God's name (Allah) so we clear any possible misunderstanding with other religions' gods

والله أعلم