Each plastic distributor has their own variation, but it’s generally
called acrylic cement. Very, very thin, looks like water. You swipe
it on with a brush, put the parts together, and it dries in seconds.
It’s pretty much just a solvent, doesn’t have any real binder to it.
(And the fumes are impressive.)
One link here:
A different version:
You can get it at the plastic distributors, but it’s not carried in
’normal’ stores. It gives you an optically clear joint. (and only
one shot at getting it all together.)
For the love of your brain cells, use it in a well ventilated space,
with a fume respirator.