With resin 3D printing, super glue or just the resin itself can work best. To use the resin, you will likely need a UV light to cure the parts together. For FDM prints super glue or a contact cement can work very good.
Some of the types of PLA plastic I use can hold paint okay, but it is always best to prime a model before painting. My favourite primer is: Rust-Oleum: 2 in 1 filler and sand-able primer. It helps smooth out some of those 3D printing lines. Sanding (and wet sanding) further helps to get the perfect surface you want.
I design my models in Blender, which is a Free open-sourced program! ( www.blender.org )
Other programs I use on tablet are IbisPaint and Nomad For 3D printers, I use Prusa MK2s and MK3, Ender 3 pro, and for resin, Anycubic mono x and M3 Max.
I do not currently offer commercial licenses to sell my products.