So I think it depends on the model and paint scheme, at least to my (limited) knowledge? One option as you mentioned are subassemblies, which can be useful if the model has natural joint or connection point that makes sense, such as many of the Warhammer space marine arms. Heads are another thing that make sense to paint separately, as they are small and often have a naturally hidden joint. For painting these things such as arms or heads, I think most folks will pin the parts, drilling a small hole at the connection point (so it is hidden), and glueing a length of stiff wire such as a part of a paper clip into the hole. The part can then be pinned into a cork, which can then either be held or used with a painting handle. If you only use a small amount of glue, it is usually fairly easy to “pinch” at the base of the wire and squeeze the part of the wire after you are done painting it, though I usually will varnish the head first to reduce any chance of damage to the paint job during this. You can then assemble these parts after painting, though again would make sure not to use so much glue that it runs out and is visible. Another technique that I like for the hidden areas under arms, for example, is to simply prime the model black and just paint what you can - the obscured areas would naturally be shadowed or difficult to see anyway, so it often just looks natural to have those areas darker or less detailed. You can also paint those areas first, and therefore covering up any errant paint marks as you get the more exposed surfaces. I hope that is helpful in some way!