How can I remove data from my iPhone remotely after giving it away?

ive given an old iphone to my cousin who lives in another country, when they charged it up it appears to be still connected to my account, im reluctant to hand over my password as all my data will still be on that phone, inc pictures etc, I’ve looked on my apple account and see the phone still in account, I can remove it remotely I think, but how can my cousin still use the phone, or do I need it to be returned for me to do it.iPhone XR

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