What comes before active boxes?

So I'm playing with the idea of getting active boxes, which I don't know exactly yet, but I've basically found a possible problem. I want to buy normal active speakers and an active subwoofer that takes care of the deep ones. These 3 speakers could each be connected to a modern PC sound card. But then I only have the PC as a source? And I do not want to pay attention when buying the boxes that the Bluetooth, cinch, jack and what do I know. That's why I got the idea that there's definitely a part of the house that has many entrances, e.g. For laptop, PC, electric piano, CD player, television etc. And then outputs in the form of the best left, right and one more for the subwoofer e.g. Sub / center. What is the name of a part and is it suitable? I've already looked around at preamplifiers, mixing consoles etc. But can't find anything. If you use passive boxes you have an amplifier with 5-6 inputs. But you don't hang an amplifier on active speakers and I don't want to buy super expensive mixing consoles. Especially since such a sound card costs 30 euro and accepts PC and microphone. So what do I need? I would also like to be able to adjust the volume best. So like an amplifier only that it does not amplify because the boxes are active. But with volume wheel, 3 outputs for L R Sub and many inputs that I need.

For the division between satellite boxes and sub you need an active crossover. However, this usually only has a stereo input.

If you don't always want to reconnect the cable, then you need a mixer or a signal summator in front of it, which leads all the different inputs together into the crossover.

Yes, it's not that elegant. But the active boxes are not so much designed for home and hi-fi use, so things like Bluetooth are rarely available.

But thanks again to the soft one, all boxes would be active and the bookshelf speakers would play 75-20k and the subwoofer 40-separation frequency. A friend said that even if they are supposed to work together, they should send the signal into the subwoofer and go out at the high cut where only 80-18k comes out and that goes into the bookshelf speakers, so they don't get anything deep. But would it actually have a crossover either way because it is active and has specified a frequency response? And with some bookshelf speakers there's also a sub out if you want to add a subwoofer, what would be the better solution and why? Into the subwoofer and from the high cut of the subwoofer to the shelf boxes (80-18k comes from the high cut) or prefer to buy shelf boxes with a subwoofer out and then there the sub ran?

No, not all active boxes have a built-in switch. There are, but not all. The frequency response is something else. This is not a cut, but an indication of how linear the playback is. If you are interested, just read about it.

If your sub has an extra output for tops and only sends part of the frequencies to it, then it has a crossover built in. Great thing, then you no longer need to buy external.

It doesn't matter whether the crossover is external, built into the sub or built into the tops. In any case, more than one is not necessary. Since your sub already has something like that, you do not need to pay attention to it with your tops (shelf boxes).