It's actually quite simple... once you know it.
3V3 means just 3.3 V, same goes for 1V8, so it's really nothing more than a shortcut. Dropping the decimal separator and use the unit instead. Safe a character or two.
1k8 is likely referring to a resistor, its resistance being 1800 ohms or 1.8 kOhm. So again, while horrible in a physicist's eye, a simple short notation only. In schematics the components, i.e. the resistor, can be clearly identified by its symbol, thus you already know that its characteristics will be measured in ohms, no need to repeat the unit in writing. Not to mention that in ancient times there was likely no "ohms" character in the limited character table... and in it were it would still print out wrong on a different system. Same goes for inductors and capacitors, e.g. you will find capacitors labelled with "100 n" when referring to a 100 nanofarad cap.
In real life dropping all unnecessary characters might have proven necessary when printing electronic components, well at least back in the olden days when these components still had a significant surface to print on.
There is probably a question to that end on the electrical engineering stack.