When you plug your guitar into input 1, its arrow contact with ground is disconnected, leaving the 1 Megohm resistor as the only direct path.
That's a million ohms "isolating" or "connecting" your sensitive guitar signal with ground, whichever way you want to look at it.
So your high-impedance guitar signal will not be all lost by finding an "easy" path to ground, and a nice strong signal appears at the input grid of one of the triode structures found within the dual-triode 7025 tube.
The 7025 was simply a 12AX7 that had been built for lower noise & microphonics, or selected for it, and given the 4-digit military-industrial tube number instead of the 12AX7 nomenclature recognized by consumers. You need low noise in the input stage more than anywhere else.
Follow your signal carefully and you will see that it has to pass through some resistance before it hits the grid, from input 1 both 68K resistors are in parallel, resulting in a 34Kohm working connection between your guitar and the grid.
Well that 34K is almost nothing compared to the megohms of internal impedance of your magnetic pickup. Once some waveforms have made it out of that highly-coiled super-long thin-ass copper wire inside the pickup, which makes your pickup have orders of magnitude more impedance ohms than its measurable DC resistance ohms, 34K is not even a speed bump.
Might as well be zero ohms and it sounds about the same but tubes kind of like a little something on that pin for stability.
Pedals can be good for more than one reason, like simple boosters that are perfectly clean and put out no distortion of their own but overdrive that input triode to complete metal mayhem, or sophisticated pre-processors with no additional gain at all. However, your signal will usually have a lot lower impedance coming out of a pedal into the amp, and so the interaction with the grid will be quite dissimilar in some ways, for better or worse.
Plus that's usually adding many more components between your instrument and that grid, and some of these solid-state components can be so tasty you don't want to do without them completely, but when you think about it, sometimes a single pedal will have more components between your guitar and that sensitive grid, than there is between that grid and the speakers. This is a legendary top pro Twin Reverb, and look how simple the rest of the vintage circuit is, before your music goes through that monster high-voltage audio transformer and hits the speakers. The tubes are doing a lot of work here.
Remember the Twin Reverb became the choice of pros and stayed that way for years before pedals were even invented.
IOW when you plug straight in is the only time you are "magnetically coupled" with the first grid, aye to the vacuum of space.
There is no further contact with what most people call "matter".
And from that point on your music is being sprayed through space, directly from your fingertips like no other way ;)