It depends on the amp as to how they do it, but the end result is always an attenuated input signal. The "high" and "low" inputs, or 1 & 2, or however they're named have different impedances. These different impedances affect how much of the guitar's tiny output voltage hits the first tube stage.
Generalized history lesson if anyone's interested:
Amps in the very early days had multiple inputs, usually for instruments and microphones. A guitar player could sing and play through one amp. The two channels ran in parallel inside the amp and got mixed together downstream. In general though if you plugged a guitar into the mic input it would distort wildly because mics are so weak and their dedicated inputs had extreme sensitivity. A guitar would send that gain stage into distortion. No one wanted that back then. The folklore goes that Leo Fender made the two input idea the way they are now so players could use single coils or humbuckers. Leo wanted his amps sounding very clean. Clean and loud was Fender's goal. Fender guitars into Fender amps for the clean chimy Leo Fender sound. But Gibsons were hitting the scene hard at that time and players were using P90s and these new pickups called humbuckers too. Humbuckers would easily drive the first stage into distortion so he tweaked the multiple input setup to where the second input was attenuated and humbucker users could use that input so the signal wouldn't distort so easily. It turned out that guitar players were turning on to the distorted sound and they liked driving that first input hard so the "low" input kind of became redundant for most people. In hindsight it's one of the few things Leo got wrong. Leo fought and fought against distortion. The players wanted more and more of it.
And then here comes Marshall who just copied the 5F6 Bassman...along with it's high/low inputs. Vox too. So now you had big-name amps all over the place with high/low inputs and it just became the norm. Marshall did something different though because they had to - they used different tubes. Higher gain tubes. Higher gain rock and roll guitar as we know it was born. The earliest Marshalls and late 50s Bassmans were basically the same amp, but Marshalls were louder and nastier because they used 12AX7s in the preamp as opposed to Fender's 12AY7.
Marshall took it one step further later on in the mid 70s with the 2203/2204 master volume amps and the inputs played a role. They took the often unused "normal" channel on Plexi/Bassman style amps and stacked it into the bright channel circuit as another gain stage...a cascaded gain stage. High gain amps were born without adding tubes and a bunch of other shit. The high/low inputs play a big role in this set up. You plug into the "high" input, the guitar signal hits the first gain stage, and then it routes back to the low input. Since you're plugged into the high input, the low input jack is closed and the signal slides right on through it to the "preamp volume" control. From there it's sent to the second stage and on to the rest of the amp. The thing with this setup is the low input is way way way less gainy than the high input because the low input comes after the first gain stage. It's not just an impedance thing, it's bypassing an entire gain stage. You plug into low and you go straight to the preamp volume pot and then to the second stage. The difference between the inputs is enormous on a JMP/JCM 2203/2204. The low input is crazy clean while the high input will rip your face off. A lot of "pedal guys" prefer the low input. It's much less loud and wild and they can get their tones through pedals or whatever.
Back to my amp, I think I'll keep the two inputs per channel because I do plan on actually using the bass channel as a bass amp. I'll probably like the low input on the bass channel for bass guitar recording.
..... just farty and pitchless on bass to my ears although at low recording volumes it would be fine..