I'm going to go against the crowd and say that I prefer DigiKey and Mouser's sites over McMaster. The filter/apply pattern they use when trying to narrow things down is a lot quicker than waiting for Mcmaster's auto updating window. Usually, when I'm looking for something, it's not for an exact specific item, but to know what options are even there in the first place. Selecting ranges of things in McMaster has always felt a little cumbersome, but Digikey has always had it right.
The other thing McMaster does that's kind of annoying, but also kind of funny, is that they go out of their way to purge the branding of the items they stock. Very understandable why they do that, but sometimes they do it when it doesn't make sense. Want to buy a generic "graphing calculator" for $126 which is definitely not a Texas Instruments TI-83 Plus? Here you go! [1]. Look, you're not fooling anybody here.
The calculator is an extreme example, but I've wondered in the past if the reason they scrub everything is so you can't take the manufacturer part number to buy elsewhere. McMaster is undoubtedly more expensive in many cases, but the service they offer is consolidating a million parts into one catalog with CAD drawings, specs, etc. Hiding branding prevents you from taking advantage of that without making a purchase.
I spoke to a McMaster web team member at a bar. They told me that the real reason there's usually no brand information is that they buy the same bolt (for example) from many different suppliers to guarantee availability.
They will only put a brand on a product (example: 3M DP420) when it truly comes from a single source and has special meaning/implications.
That said, I order tens of thousands of dollars of McMaster Carr items each year. They almost always come in packages from the OEM with OEM part numbers. So if I want more bolts like that, I just look at the box they were delivered in. The info is just not on the web interface.
It's pretty much the same business model invented by the Sears Roebuck Catalog. For many years everything was pretty much unbranded, then they created "White Label brands" like Craftsman (and a few others) which grew to become standalone consumer brands which have outlived the parent.
I think everything McMaster dates to it being a physical book first. They still operate on that same business model, but we have the internet now. The supplied product might change, but it still meets whatever specification is in the catalog that is released yearly. If they could guarantee a TI-83 Plus was what you were going to get they would have put it in the catalog, but they couldn't so they don't. And they STILL operate out of that physical book for some customers, so the website has to match it too. That's my take.
Oh yeah, the books are impressive in their own right. However many items you think they sell looking at the website, the book reveals you were a couple orders of magnitudes off. 1000+ bible-thin pages of well laid out tables and product photos. It's pretty much what the website would look without any filtering of the items.
The books are fun to leaf through on occasion, or if you need to take up an extra 3 inches on your bookshelf with something yellow. If you have one, it makes you feel like a real engineer. But I greatly prefer the website.
I kind of like how they genericize everything. It reduces the cognitive load of making decisions, and presents all of the options in the most uniform way, based on their hard specs, and not marketing BS.
It's ok most of the time, but it's not like they sell McMaster branded stuff in McMaster packaging. They are a supplier, and might want to obfuscate to keep you from buying direct from the actual manufacturers.
On the other hand, on my desk right now is a bag of springs, the info printed on it says it was made by WB Jones, part number 4011. We ordered it from McMaster. Why not? They stock the item and ship super quick. If I want another bag of the same springs, it's not like I can go to McMaster, type in "springs 4011" and expect to find it. Instead, I'll have to search up purchasing requests I've made, maybe ask a coworker if they ordered them, etc. to find the mcmaster number again. If I didn't know Mcmaster sold it to us, I'd have no idea they sold it at all.
To be fair, if they sell things that are interchangeable, like screws, it would be a lot to list every manufacturer they use. They have 5 locations, and probably stock from a different manufacturer or multiple manufacturers at each one.
Buying from McMaster is like ordering mil spec part numbers. You don't really care who makes it or what vendor-specific p/n they give it, you just want the part built to spec for the cheapest price and soonest delivery date.
The other thing McMaster does that's kind of annoying, but also kind of funny, is that they go out of their way to purge the branding of the items they stock. Very understandable why they do that, but sometimes they do it when it doesn't make sense. Want to buy a generic "graphing calculator" for $126 which is definitely not a Texas Instruments TI-83 Plus? Here you go! [1]. Look, you're not fooling anybody here.
[1] https://www.mcmaster.com/8392T11/