The Heathkits and even things like the Radio Shack spring-wire 250-in-one type kits had you build circuits from all individual components: resistors, capacitors, diodes, transistors, inductors, etc. If lucky, maybe a few basic ICs like triple-nickels, op-amps, comparators. After hours of soldering/wiring you had something that did some very basic function - not a microcontroller/SBC with full programming language support and more power than a supercomputer a few decades ago.
In 1980 I got one of these kits for my auto repair shop, and yes, it was quite a challenge to build, but it saved me hours of automotive diagnostic time.
A comparable automotive ignition analyzer scope at the time was thousands of dollars.
Heathkit CO-1015 Oscilloscope Engine Ignition Analyzer Tool
https://www.ebay.com/itm/144853683301?epid=1323946549&hash=i...
Jameco Electronics https://www.jameco.com/
The Electronic Goldmine https://theelectronicgoldmine.com/
All Electronics https://www.allelectronics.com/
Back in the 60's through the 90's we also had Haltech and Halted Specialties, electronics supplies and surplus in Sunnyvale/Santa Clara, each about a mile from each other.
Many folks confused the names.
As I recall, both of them had resident cats as well.
My High School electronics teacher turned me onto them in 1969.
It appears Haltech is defunct, and not surprising since the owner was OLD when I was shopping/browsing there in the 70's - 80's.
Oh, looks like Halted changed their name to HSC and may still be in business.
https://www.scrapmonster.com/company/hsc-electronic-supply/3...
Aside from many parts, I also bought my first of several surplus HeNe LASERS there in 1974.
Back in the day, business computers were the most powerful systems. Nowadays, it's consumer electronics that is getting more powerful. Which is great for consumers. But not so great for startups who want to build their company on top of these systems.
Nowadays it's mostly reversed, so many gadgets can be just bought for cheap, hell even if you want DIY what you will DIY changed massively.
Back in ye olde stuff like signal generator or power supply was probably the stuff you DIYed or made from kit, nowadays they are so cheap and so much better than whatever you'd make first. On flip side dev boards or breakout boards with various "fun" devices are aplenty so your first projects might be something actual EE might've spend week to build in the 90s
The "everything is a breakout board" might make people not start with the basics at first but in the end I think that lower barrier to entry is a good thing here
"back in the day", it was hard to get anything, computers were expensive, electronics (except very basic) were hard to source, and prebuilt software (for whatever custom use) was non-existant.
Now, a microcontroller with wifi and bluetooth is <$5, a computer/server for running code is ~$50 (if you manage to find a raspberrypi, or a bit more for a micro pc), wireless interfaces (zigbee, bluetooth) are cheap, prebuilt code for sensors (tasmota, esphome,...) is very mature and stable, central management systems (home assistant,...) are stable and support pretty much everything...
The only problem I see is, that a lot of people have lost their tinkering capabilities and don't want to get their hands dirty compared to "back then" when a lot of stuff needed getting your hands dirty and more people went along the DIY path.