Tweeters with or without fluid?

No fluid at all!  Ribbons are much faster. Catchy nickname btw.

The main function of the fluid is to cool the voice coil of the driver by transferring heat to nearby structures.  The fluid can also act to damp the motion of the coil/diaphtagm.

The main downside is that the fluid will dry out in about 15 to 20 years.  With some drivers, the gummy remsins of the gluid can be cleaned out and replaced eith hew fluid.  I have not done this so I don’t know what that entails, but you can look it up by googling speaker ferrofluid.

Ferrofluid can help flatten the impedance curve and reduce Fs and diaphragm excursions, but each design is different and handles those potential issues differently, so it’s use isn’t universally advantages.

I changed out the FF on my 30 year old Seas domes a few years ago, and it made an audible improvement after.