I'm thinking of aliens that don't have hands but use their tentacles to make and use tools and, eventually, advanced technology. I would like to know whether this is a realistic idea. For the purpose of fine manipulation, what are the advantages and disadvantages of tentacles compared to humanlike hands?
The problem with tentacles is that you need to make massive changes in human physiology.
For example octopus, squid and other mollusks are invertebrates, creatures without bones adapted to live in bodies of water. If humans had tentacles instead of hands it would be ineffective because they would probably have bones limiting their movement.
The tentacles with bones, if they were broken, would leave their user disabled for life, especially in ancient times without medicine or surgeries. So if your humans somehow evolve flexible bones like the sharks it would be possible.
A) Consider the human tongue; a long boneless appendage, muscular, that (in some people) can manipulate finely enough to tie a knot in string (perhaps holding one end in the teeth, I don't know).
B) Consider the nose of the elephant; the tip is bifurcated and can be manipulated by the elephant like fingers: It can pick up a dime off of a smooth floor. It can use the same trunk to pick up a log weighing several hundred pounds. It can pick up a child and put it unharmed on its own head. It can pick an apple off a tree.
C) Consider the octopus; it can open a screw top jar using two tentacles, and reach inside to capture a shrimp with a third.
I don't think there is any issue of fine control. I do think, of course, our tools and technology are themselves 'evolved' to fit what our hands and joints can do. Our buttons and handles are a size made for fingers to push, or hands to wrap around. How a tentacle might be used will be different: if your buttons were on a device that fit in your mouth and were meant to be pushed with your tongue, the device would obviously look much different than a keyboard; require different tactical feedback, and perhaps be more sensitive to a push.
If I need to hold a tool, we all know how to make our tongue large by contracting muscles so we couldn't even close our mouth: A tentacle might have fine control of the tip to allow the same; so the 'handle' of such a tool is an empty cavity, into which the tentacle is inserted and then muscularly 'inflated' to 'grip' the tool from the inside. certainly those muscles can be as strong or stronger than human hand grip, of a hammer, screwdriver, etc.
Speaking of tools, I'd expect little variation on what we have. A screw or bolt is an inclined plane in helical form; it is a useful shape that has nothing to do with human form. Many tools are the same; a hammer isn't about humans, it is just a refined version of a hard rock; a saw or an axe or a blade or a drill has nothing to do with human form, neither does rope or string, gears or pulleys. An elephant can pull a rope over a compound pulley with its trunk, even better than a human can with its hands and arms.
It isn't the working end of the tools I'd expect to change, just the 'user interface' if the engineers have tentacles instead of hands.