AdaFruit and SparkFun both provide MCUs, sensors, and other peripherals that integrate well. Couple that with copious libraries and example projects and you may be up and running without having to stare at data sheets and wiring diagrams and JTAG output just to (say) get a temperature reading and display it on a tiny OLED screen.

All of that plus maintaining inventory nearer their customers, doing effective QC on units they ship, writing good docs, etc. means you’re getting something a lot more like a “big OEM” experience from the hardware vendor, even if you’re ordering a handful of parts.

The generic AliExpress vendors, in my experience, do not do most of those things. They all support Arduino and/or PlatformIO, and sometimes a “native” SDK like mbed, but you’re often on your own figuring out how to integrate that bare MCU with other devices you need for a complete solution. Docs are often incomplete or untranslated, and it can be hard to know exactly which chip (or associated components like onboard sensors and BME) is on there. It can change between board revisions, or even identically-named parts from different vendors.

There are other players like M5 and RAK who make nice modular platform as well, but their prices tend to be up there with AF and SF.