I’m going to assume you’re using wired headphones and earbuds:

Every device will have a different sensitivity - IEMs, headphones, powered speakers, passive speakers, etc. and it varies wildly between different IEMs, different headphones, and so on.

That means every device requires a different amount of power to drive it. Your PC doesn’t know what device you’re using - it just outputs the same amount of power at the same volume level, and it’s up to you to adjust it to the appropriate level when you’re using a different device.

Now, different output jacks on the PC may output different relative levels even at the same volume setting. The headphone output will have a bit of extra power driving it, raising the level to drive passive wired headphones and earbuds, while the stereo output will have less power because it’s assuming that your speakers have their own separate power source - either built into powered speakers, or with a powered amplifier in the line between the PC and passive speakers.

Assuming you’re using the same jack for the earbuds and headphones, the volume difference is because your earbuds are more sensitive than your headphones. The earbuds require less power to drive, and the headphones require more power to drive. Windows simply spits out the same amount of power to the headphone jack at its volume setting, so the earbuds are louder and the headphones are quieter, even though they’re receiving the exact same amount of power.

If you don’t like adjusting windows volume, one solution is to get a USB headphone amp. Great headphone amps can be had for around $100 from brands like Topping, JDS Labs, Schiit, and more. It’s a competitive market so there are tons of options. This won’t even out the levels between devices, but it will put a nice volume knob at your fingertips, along with the jack itself for switching devices. Many of them also reroute audio to your speakers at the touch of a button. You’d then simply “mix” the volume level on your speakers to match your preferred knob position. Another bonus is that there’s no analog audio signal swimming around with the other noise in your PC, as it’s now leaving digitally over USB, so the audio quality is usually noticeably cleaner.

There are some old school and professional headphones that require headphone amps to drive, because even at 100% windows volume, most PCs will simply not output enough power. Whenever you’re shopping for passive headphones, you might see reviewers say things like “these headphones are easy to drive” or “hard to drive” - they’re referring to how much power is required to achieve acceptable listening levels, and advising whether you’ll need a beefy amp or not. Most consumer headphones these days are designed to be driven by weaker power sources like those found in phones and PCs. But again, it varies wildly - and what you’re experiencing is a small variation in power requirement between two devices.