Hi SpaceJeff,
Welcome to SDC.
Venus unlike Earth appears to have a single layered core (the Earth has an outer & inner one, Venus appears to have just the one).
This core could well be molten, but is not convecting, hense lack of global magnestosphere. Also the Earth, Mercury & the Jupiter moon Ganymede have inernally driven magnetospheres & all three appear to have dual layered cores, something that Venus & Mars boh lack. To me this seems to be more than just a coincidence.
Perhpas a differentiated core like Earth's or Mercury's is composed of a solid inner one made of Iron & Nickel, with a moltern outer one made from Iron Sulphide, which is churning over.
In the case of Venus, this lack of core differentiation has probably ceased any global magnetospheric development. It does not preclude volcanism as magmatic Hot Spots can still rise through the mantle & erupt through the crust. True Venus currently lacks Plate Tectonics, so is in effect a 'One Plate' planet.
As you correctly say, much of the Earth's volcanism is on plate boundaries, either in the spreading zones like Iceland, ot the Azores or subduction like The Cascades, The Aleutians, or The Andes, or The Phillippines, or Japan, etc.
The Earth also has some examples of Venus type hot spot volcanism like Hawaii, The Galapagos Islands, or Yellowstone, but these are pretty rare on Earth, but is common on Venus. Lack of global magnetosphere & undifferentiated core, does not rule out volcanism provided there is enough internal heat. Whilst the structure is different, the internal heat budget is similar to Earth's.
The main difference is though, that Earth gets rid of most of it's internal heat through Plate Tectonics. Venus appears to 'bottle up heat' then releases it through mass volcanism that resurfces the planet in episodic mass lava outpourings, though Venus appears to have lesser individual hot spot eruptions in between (based on what is on the surface).
Remember Venus has 81.5% of the mass of the Earth, so will still be hot inside from it's formation & also Venus will still have radioistopic elements internally. Venus's density is not much less than Earth's either, 5.250 GCM3 (Grams per cubic centimetre) as against Earth's 5.517 GCM3, suggesting a very similar make up. Both planets are compressed internally due to their mass & resulting gravity, giving both uncompressed densites of approx 4.6 GCM3 (Mercury with a mean density of 5.43 GCM3 is also the uncompressed density making Mercury the densest planet overall).
Also another object that also lacks a global magnetosphere but is volcanically active is the Jupiter moon Io, though that is due to tidal heating, but that's another issue.
It is late here & I am tired, so hopefully my answer makes sense.
Andrew Brown.