Internally, the 99 is more similar to the 226 than the 88.
Disassembly of the 99 is the same as for the 226, i.e. lock the slide back, rotate lever, release slide (off). For the 88, the disassembly lever can be activated with the slide fully forward.
The disassembly lever for the 99 and 226 retains the locking block that houses the disassembly lever retaining spring, provides a small feeding ramp to the barrel's feed ramp, covers the trigger assembly, and most importantly, provides the camming action surface for the barrel during cycling (to tilt downward). This block can be removed buy pushing the retaining spring out of the way by rotating the lever further and pulling it out. The 88 does not have this.
The firing pin of the 99 and 226 work as normal with a firing pin stop. (Going by memory here, don't have my 99 with me at the moment.) The 88's method of firing pin safety is to lower the entire firing pin assembly down to align with a recess in the hammer. This recess prevents the firing pin from being struck in the event the hammer inadvertently falls without the trigger being pulled back.
The 99 and 226 do not have an adjustable trigger stop.
The action bar of the 99 is similar as the 226 being internal and disconnected by a notch on the inside of the slide. The 88's bar is external ala P38/P1/Beretta/Taurus.
Although all used the modified Browning action, the 99 and 226 have parts, e.g. barrel, locking block, etc. that look nearly identical.
The only thing that the 99 has in common with the 88 is the ambi slide stop/decocker and the ambi mag release. Other than that, it's essentially a 226.
As noted, the Bersa Thunder9 is more akin ot the 88 than the 99 (or 226). There are some differences between the two, though, but that's another story.
I don't have all of these guns with me at the same location at this time but if you can wait until Friday or Saturday I'll post pics (including the Thunder9).