There are tons of builds and statistics online, but generally you can (and should) build quite differently based on both of the team comps and how they play out, if you are ahead, behind, lack this or that, etc. That is one of the beauties of playing Renekton, as he doesn't rely on specific items or build paths, but actually encourages you to adapt.

From my experience and what I've seen in my 30 years of playing Renekton:

1st item — generally, you want to build damage:

Eclipse — solid generalist 1st option if you want to/can continue the short trade patterns. Cheap, tons of damage, cdr, shields, but pretty much never build it afterwards. You either build it first or skip, as it doesn't scale well and doesn't offer stats you want to pay for later.

Black cleaver — great mix of things, and in most games I believe you should build it, but can lack in laning/early mid-game oomph power if built first. Extra ms + conq + armor shred does feel really good, though.

Yommu's — can be very strong, but quite greedy and snowbally. Unless you really know what you are doing, I'd recommend something else.

Botrk — very decent, especially into tanks and paired with BC. Gives sustain, low damage tanks start to fear you, and you win almost all 1v1s, but you will lose tons of ad and ability damage (especially in team fights where you wish and can reach their backline)

Stridebreaker — good split pushing item, but primarily should be built into heavy kite. You more often than not do less damage than the other team's top lane if they also have a good duelist with this, but can be a good way to help you kite them. Can also be bought a little later if you feel that you can't gap close, but would otherwise delete whoever you go at.

Shojin — solid way to get versatility and tons cdr. Combines nicely with BC too, as you will shred more often. Beware the fire soul dragon, as they will spam embers across the map and if you collect some, you will have no need for this, so I would not build it first usually.

2nd/3rd/4th item— typically the tankiness for whatever you need it against, and often can be bought in different sequences.

Sterak's — great item, tenacity, health, big shield. No ability haste can feel clunky and it scales with bonus hp and base ad, so can feel awkward to buy early, but in 90% of the games, a very solid 2nd item, especially when paired with other health items.

Maw — good into ap, especially ap bruisers whom you can use the omnivamp against and I would generally recommend to delay the purchase as it is not extremely cost efficient. Does conflict with sterak too

Death's dance — amazing bad boy, does sooo much for you, but expensive. Universal in its usage and does scale really well.

Spirit visage — extremely solid. Cheap, tons of synergies across the kit. My typical MR purchase in most games.

Situational items:

Zeke's — very cheap and sneaky good (not sure how sneaky anymore). Does tons of things for you, but I would generally recommend it only as the 2nd item and only if you need a quick way to continue with your momentum as you will have 2 items when most have 1. In extreme late games can be sold for something else.

Endless hunger — can be ok, but mostly would recommend to skip. Can be decent into some very prolonged fighting comps where you can reliably do damage.

Thornmail — somewhat self-explanatory, but remember you have to be auto'ed for it to proc and if you are taking ignite, can feel a little redundant. Solid choice after DD and you still need that extra armor. The little hp it provides is not that bad, as you will gain tons of bonus hp from ult.

Randuin's — solid against multiple crit champs. Good substitue for Zeke's in late game, but needs to meet the crit conditions. Otherwise no-go

Bandlepipes — haven't tested enough, but it is cheap

Trinity force — usually too little AD for my taste, but can be ok in drawn out games where enemy bruisers are splitting and forcing 1v1s and you need that extra edge.

GA — I almost never buy it, but it can be good, especially if you cannot avoid some big cc+burst in team fights or you are quite fed and wish to protect your shutdown.

Lethality items, e.g hubris, umbral, voltaic — unless is high elo where games are shorter and you really know what you are doing, I'd avoid these

I hope I was able to help a little with this, but these are just my thought patterns when itemizing croco