There are numerous bluetooth headphone adapters available on-line, from about £20. They have a 3.5mm socket for headphones, so all you need is a 3.5mm - 3.5mm cable. The cheaper ones don't have APT-x compression, so it may be worth spending a bit more, as the technical quality is better, but for casual use, a cheap one may be good enough.
It's not bluetooth, but the best adapter for sending audio from an Android 'phone to hifi is a Google Chromecast Audio. They were discontinued some time ago, but are still available on-line. Technical quality was superb, but depends on the application as to whether it would be suitable.
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