How do you use ICs in your own projects that don't come with a datasheet?
How do you use ICs in your own projects that don't come with a datasheet?

I don't.  That would be irresponsible engineering.  Putting something in the field that you don't really know all the specifications and requirements of is very risky.

OK, so you get the prototype to work on your bench.  That only means you got that one board working.  Maybe you tested it over a range of supply voltages, but what about temperature, humidity, certain sequences that may require specific timing to other things?

The real problem is you can't even know all the parameters that matter.  And even if you did, getting a sample of one to work only means it's possible, not that it will always work.  How do you know that your unit is a little more tolerant of some key parameter that you happen to be on the edge of?  And remember, you don't even know what all the key parameters are, so you can't say you've added margin to everything.  Even then, how much margin is enough to account for unit to unit variation, variations over time, temperature, humidity, vibration, handling history, etc, etc, etc.

Worst case you're close but not quite there.  The first 20 prototypes you send to beta sites work great.  They only had a 1:5 chance of hitting the 1:100 error, and this batch didn't happen to hit it. 

After you have 1000 units in the field, you've got 10 customers all around the world complaning that your crappy box doesn't work.  The other 990 customers are fine and don't make any noise, but nobody hears them.  Those contemplating buying your product see ten separate bad reviews, and immediately cross your company off the list.  This will stick in their mind, and they won't buy other products from your company either.  Of course nobody tells you why they skipped over your product, they just silently buy someone else's product.  Then two out of the ten customers with bad units go out of their way to make a stink on the internet.

If management is clueless, the company will slowly coast to a stop and you lose your job.  If management has someone competent getting to the bottom of this, you'll be fired for incompetence and you still lose your job, and a track record of irresponsible engineering follows you around.  You are forced to find a new career and now ask "Do you want fries with that?" for a living.

Don't be that loser.