All grounds on the pedal/whatever are "THE" ground of the circuit, the idea is to have at least 1 reference point of V=0 (because the mass which creates the ground is so tremendous that the electrical current "diluates" there, "dies" and if all your "V=0" points are linked to the same mass, then you know for sure their tension is 0). On a car, ground could be the car structure itself, etc.
But on a schematic, you just draw the ground connection wherever you need V=0, and basically you physically link this point to anything that is considered ground on the product/schematic. Drawing this specific physical links on the schematic would be too complicated, cause they physically link to the same point usually (on a car : its structure). To avoid drawing useless lines (because ground is trivial), we "don't care" about it on the schematic, and just draw the ground logo wherever it's needed.
So it can be a long line, or the PCB can be made so that you have a line on one of its side that you decide will be the ground (and it's usually there for that purpose), or you solder to anything that would make a ground.
I'm not a specialist in analogic circuits (i was more into programming circuits or drawing schematics, and have the computer solve the actual result with all the chips and stuff) but it's how I managed that when I had to read them.
On the same note, whenever you read +VCC -VCC (or V+ V- or whatever) or +Vr -Vr, you have to link these points to where the + of your AC alimentation is or whatever is needed, you can link that to the actual + of the alim, or create a line from there, or whatever, as long as its potential is +Vcc, but on the schematic we don't picture that, it's up to you and it's irrelevant in the schematics as well, it's basically to say "here you need the + of the alim" and they are not gonna draw a line from every one of these points to a reference +Vcc point, it would simply add a dozen of useless lines that would make the schematics unreadable.
About actually read a schematics, there is no secret, when you open your pedal it's a mess but it's like that, and the more complicated a circuit is, the less you can recognize it directly on the final result (here it's not complicated, but sometimes to be able to overlap two lines you have to manage the two sides of the PCB, and there are current hi tech circuitry who go "3D" and have several layers of lines, so you basically cannot humanly read them. Someone well trained will be able to understand analog circuitry though, because you can recognize blocs which have a function. Like you can recognize the preamp, amp, alim, and equalizers on an amp schematics. Also, some associations of elements (between resistors/caps/inductances - sorry i don't know the words in english) have basic goals. A high pass filter or low pass filter are nothing else than an association of resistor and cap or inductance and their -3dB frequency is RC for example, but for that you need to understand at least a little the maths behind that.
On a more "human" level, you have to understand the duality of C and L. I'm gonna exagerate to the extreme characteristics, it's how I used to do to read the schematics, by exagerating their impact and the conditions of the circuitry (e.g. "if frequency is infinite, what can I remove in the circuitry and then how can i understand it")
- in DC current (frequency 0)
caps block the current (the charges accumulate to their plans to a point when they saturate)
inductances induce a delay but eventually would then have no effect when the current is fully established
- in fast AC current (frequency infinite)
caps have no effect, since the + and - invert before any accumulation has been made on its plans
inductances block the current cause there is no time between alternates of + and - to "win" against the delay, the inertia
and then for mid frequencies, the effects are in between. Bu when you get that, you can undersand why some C or some L are there, why this part is a low pass, why this one is a high pass, it makes a lot of things easier. And it's actually a specific technique of reading some circuitries. With precise initial conditions, you can redraw the circuitry step by step by removing or changing components to their "equivalents", and in the end you have only a few, if not 1 component, with the characteristics of the whole circuitry in those initial conditions. The duality I mention is interesting because from one extreme to the other, you simply invert C and L, low pass/high pass, and other opposite things, and with a change of variable, you can recreate the same circuit from a DC current, etc. It's mostly theory electronics, but it's interesting to mention cause it helps understand.
You basically need to learn the effects of the different types of components and their associations, and make an analogy with a current of water, or whatever works for you. Then you can get the point of each block. I'm not even myself able to read the circuit you posted, except a few basic parts (and nothing too deep).
I have never been a specialist in analog circuitry, and I may have said something wrong somewhere cause I haven't done that in years, I left my electronic engineer for something else, so don't quote me but it's my 2c, and nothing would beat a specific course on the subject cause it's vast and not easy in the beginning.
) same for "V+" (9Vdc) and "Vr" (virtual ground for the op-amp).
