Union

Disclaimer: A copy of this game was provided by the developer.
Union is a minimal logic puzzle game.
The point of the game is to connect all the pieces so that none are left out. At first there are only rotating pieces and later pieces that are swapped to another location. The game does a good job of introducing mechanics, using them to create challenging levels until it introduces another mechanic and adds it to the mix.
There are 50 levels that took me 3 hours to complete. Some of the challenging levels take some time to figure out, but there is relaxing music to help you through them. There's a dark and light mode.
For achievement hunters:
There is one achievement for completing all 50 levels.

Union is a minimalistic puzzle game about swapping tiles in order to create connections between several shapes of the same kind. Each level features a tangled layout of pieces, and the game challenges the player to find a way to rearrange them so that all connections are satisfied.
I liked it, then I disliked it, and in the end I fell in love with the mastery behind the handcrafted levels and the way the different mechanics sustain each other.
The game starts slowly, with a few very easy levels, meant to showcase a few basic ideas: each tile has one or multiple edges with connections towards the outside; you need to move them around in such a way that a link traversing multiple tiles will always bind shapes of the same kind. Soon enough, rotating tiles were introduced: by rotating them, the orientation of the connections change, therefore the paths between shapes change as well.
The difficulty ramped up substantially and while I am not one to back away from a logic challenge, that’s the point where I started to dislike the game. The more complex the levels became, the harder it was for me to keep track of the actions performed. The levels can be approached in a very logical manner by isolating the tiles that have only one possible position, and then building up tile by tile from the obtained configuration. However, during the more complex levels, the remaining tiles had so many possible combinations, and the game slowly turned to be less logic and more “trial and error”: pick one tile that fits and continue following your logic until you hit a dead end; then go back, change it and try another path. However, considering that there is no way to undo moves or mark any tiles as “certain / uncertain”, this whole backtracking had to be done manually (for me: taking screenshots and marking the tiles accordingly). I soon realized that the problem isn’t that the levels are not properly crafted, but the fact that there’s no tool to support your logic process. Luckily, the developer responded with a positive feedback to my request of adding such functionality - I’m hoping to see it implemented in the near future.
On the positive side, there were only very few levels in which the struggle of backtracking my moves hit me. Once I managed to pass those, a whole new world opened for me: new mechanics were introduced, new tile types: shape-changers and antennas, with the latter replacing the requirement to have a “wired” link between two shapes. And that’s the point where I started to really really like the game. For me, it became much more logical than before and I began to have lots of “aha!” moments while playing it. Some of the levels were still pretty difficult, but what in the beginning seemed to be a nonsensical mess of tiles turned out to be a brilliantly crafted set of traps for the player to overcome. I loved the challenge it provided and searching for the centerpiece that holds everything together was a pretty funny experience as well, even if that meant having to “almost” solve the level fully.
With handcrafted levels complemented by a very relaxing soundtrack and a dual color scheme (light & dark modes), this little game is a must have for puzzle lovers in need of a challenge.
More reviews on the Lilly's Corner Curator page

union is a minimalist puzzler about connecting (or uniting, I guess) various shapes with the lines running between them. tiles with a dotted circle border can be rotated, right click turns them in the opposite direction, a feature most rotation-based games seem to forget about. and that's the jist of it.
there are different shapes to connect (triangle to triangle, square to square, etc.), tiles with multiple lines running through them, and ones with a dotted square around can be swapped with another one with a dotted square border. these can't be rotated, but clicking them highlights potential pairs. then there's dotted rounded squares, no rotation, nor swapping, but their contents can be changed by clicking. and finally, relay tiles ignore gaps and connect stuff in a wireless fashion.
various resolutions, windowed mode, separate volume settings and an exit button are all included, i.e. the absolute minimum for any game, yet they rarely make an appearance together. there's also a night mode, selecting levels by going back and forth one by one, and a proper level select screen with level numbers, another common omission and I totally missed it here, as you have to click the level number to bring it up.
pleasant audiovisual presentation, 50 levels, no timer or anything, but I found the black & white presentation to be more of a curse than a gift and it caused more frustration and tedium than I would've liked, as after the first third of the game things start to get real and I would've preferred to see tiles of the same kind colored or marked differently for easier recognition instead of everything blurred together in one dotted mess. it's pretty good otherwise, should take 2-3 hours and the price is the same as on mobile ($1).

Wow some of those levels were quite a challenge to solve! It's a great puzzler -- simple connect-the-dots kind of premise but introduces mechanics as you progress. Helps keep the levels fresh as you progress through 50 of them. The calming music is a nice touch as well!

This is an excellent choice for this price range. While the game starts simple, some later levels are quite challenging.

fun little puzzle game, very much worth its price for 50 levels

Fantastic logic game and for a great price! Love the simplicity of it as well.