Does it say "Samsung" on the front? I would first deal with the immediate safety issue of the combustion.
Why do phones catch fire? The primary reason would be a charging circuit failure, causing an over-current into the battery which is very fragile. Another possibility is a poor battery design or poor QC on the battery construction. The charger could also be the cause, though much of the charging regulation happens in phones these days, not in the charger itself.
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