Like this one [here](https://www.auspienterprises.com/wp-content/uploads/2021/12/PCB1-1.jpg)
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Those are passthrough holes for different components. Not all components have the same size posts. Higher amperage components need bigger posts as not to melt. This PCB has several configurations so all the through holes won’t be used. Cheaper this way.
Some are holes that are meant to have components leads through them. Others are meant to just connect the point you see to another layer of the board. This lets printed wires cross other wires easily.
Older simpler boards have just the two sides. Newer higher density boards can have several internal layers, all with their own tracks, that you don’t see.
The small ones are called vias. The larger ones could be vias as well, but they look more like holes for through-hole components. Through-hole components have leads that go through the board (imaginative naming, no?). Those silver strips on the right side of the photo are pads for mounting a surface mount component. Surface mount components just sit on top of a board and don’t go through.
Vias serve a couple purposes:
1) The connect traces and pads to layers on the other side of the board or internal layers (many PCB are composed of several layers). When designing a board we like to minimize the number of different size holes as it requires drill changes which slows production which increases cost. Because of this, we usually make vias the same size. Too small and the board fabricator will charge you extra because they’re more difficult to do and they might not be able to handle the required current, and too large will take up too much space on the PCBA.
2) The other purpose of vias is to conduct heat away from the part. As previously mentioned most boards have multiple internal layers. Usually at least one of these layers is connected to ground which is almost fully a single foil of conductor with a bunch of holes. Certain parts will have a large area underneath them that is intended to be connected to ground and to allow heat to flow out of the chip. The PCB will have a large area where this pad is soldered. In this pad you will see a bunch of “thermal vias” whose primary purpose is to carry heat away from the chip to the large conductor area.
Through-hole parts require holes of various sizes dependent upon the geometry of the lead that needs to go through them.
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