I use a fair bit of 5 minutes epoxy, and try to make the mating parts have some sort of keying and registration, with both help with alignment and with strength of the joint.
Then I go over the seam with a sharp knife/chisel to remove any squeezeout and blend it.

After that, it’s a whole lot of using putties, filler primer and sandpaper to get it smooth and hiding all the signs of it having been 3d printed and assembled.

Assuming you’re talking about FDM/FFF printing and not SLA or sintering:

There’s a bit of a catch-22 when it comes to mating flat 3d printed surfaces: The face that’s on the build plate will be the flattest and result in the smallest seam. It’s also the face most likely to be dimensionally inaccurate because of elephant footing, which can result in a ridge at the seam. And the easy fix for having an elephant foot it adding a chamfer, which results in a big visible seam. So my actual advice is:

1. get your first layer super-duper dialed in
2. then make sure “elephant foot correction” (or whatever your slicer may call it) is turned off in your slicer (or it’ll add a 0.4mm chamfer for you)
3. make sure your mating faces are face down on the bed
4. avoid textured build plates if you can

And if aesthetics are a high priority consider using an automotive filler primer (I buy it in spray cans) and then painting the piece after it’s glued. Filler primer will help hide the seam and layer lines.

I have a welding iron (for electronics) with an electronic temperature selector. It’s less than $20. I can set it to 190°C and push the PLA to hide the seam.

I just bought a bigger printer. No more seams! Realistically though, plastic welding or epoxy and paint, and a whole lot of sanding.