How to solder tiny, tight, narrow pinhole and PCB layout and..

How to solder tiny, tight, narrow pinhole and PCB layout, and what thing/material to prevent solder to go over neighboring pin/hole ?
Of course, means it must be little/non heat conductive to prevent draining soldering iron heat

Ultraviolet curing solder mask. Iron with enough heat to QUICKLY solder area. Practice on a scrap board first.

The most obvious thing is to put all the components in before soldering, if possible, so heat transfer or solder flow between leads/holes in the same copper pad does not matter.

If you must leave holes free, you can put Kapton tape over them - that is heatproof and commonly used to protect areas of PCBs during manufacture or assembly.
(Like to prevent tin flowing over gold edge connector areas).