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I’ve seen references to this in old codecs, but don’t know where to look to find them. Especially since I don’t own the old codecs.
Anyone got some references I can use? I’m curious to see what Nids used to be like.

It was only a thing in the 3rd Ed codex.

You started by picking one of the 'mutable' species- Gaunts, Warriors, Ripper Swarms, Carnifexes, or Hive Tyrants.

Then you picked biomorphs, altering their stats. Eg you could boost WS with adrenal glands, BS with enhanced senses, Sv with enhanced carapace, or give them leaping or winged movement. Then you chose weapons.

Different upgrades could shift around what slot a unit occupied. Warriors were normally HQ or Elite, but if you gave them more than one heavy weapon they became HQ or Heavy Support, or if you gave them wings they became HQ or Elites.

Once the total number of species for your list was determined- your homebrew species plus whatever 'geno-fixed' default entries you wanted (every unit besides the above five)- then you moved on to mutants, where members of a squad could have abilities like acid blood, synapse, gargantuan size, or an abnormal weapon. The number of mutants per squad was one per X models, where X was the number of species in your fleet.

The 4th Ed codex got rid of all of this and instead worked the unit upgrades directly into the codex entries. That's where the endlessly customizable plastic Carnifex came from, because in 4th Ed you didn't need three different species in order to give wildly different upgrades to three different Carnifexes.

^Confirming 3rd and 4th Tyranid books, the 4th Ed book being my favorite Tyranid book, I think. Something about the overall presentation is very satisfying.

Prior to those, in 2nd Ed, Tyranids had a list of Biomorphs a number of unit could choose from. These included some very exotic options such as "Electro Field", which gave the model/s a 4+ Invulnerable save, and had a chance of knocking out opposing protective fields. Like, a Lictor could have an Electro Field that permanently removed a Chaplains Rosarius save. But maybe more notable was that Tyranid Warrior Squads could be given Biomorphs as a unit. Like they could all have the same Warp Field that a Zoanthrope had. You could do some neat stuff.

The last option I know of is the Tyranid Monstrosities Builder, which was the Tyranid equivalent of the Vehicle Design Rules of late 3rd ed. This provided a framework to build Tyranid MCs and Superheavy equivalents. You can find these in Chapter Approved 2004, which incidentally also has the rules for Tyranid Seeding Swarm armies.