At the Leverkusen vs Roma game the German fans set a fire that was 30-40 feet wide! Ive seen this in other German games. Cameras never do a closeup but it is a spectacular sight -doesnt last long and causes
smoke that covers the field.

Looks dangerous but is obviously allowed. Anyone know how it is done without harming the fans?

The "fire" is a bunch of flares being held by the fans, usually the more, uhh, "enthusiastic" fan groups. You might know them as "hooligans", they'll call themselves "ultras." They've been used by soccer fans for a long, long time.

I think they're generally safe if you're not stupid about it, because you're just holding it up above heads and they burn upwards, which is why their use persists in a lot of countries. But you won't see them in Premier League matches if that's your general exposure to soccer, as they're banned in English stadiums.

Many thanks,TxAggie2011 !