It has nothing much to do with welding skills as such, rather that you need to weld in an inert atmosphere free of impurities by using shielding equipment, which only specialist facilities can achieve. However proficient the welding, if there are any impurities in a ti weld, it will be liable to crack in service.
As far as I am aware, Litespeed was by far the biggest supplier of ti frames to other brands, followed by TST (later TiSport) and Merlin, while A-Pro was the leading ti welder in Taiwan. Most cheaper ti frames were welded in Russia by anonymous companies.
Litespeed made Marins until 1995, when they shifted the contract to TST. My 1995 KHS TiLite was supposedly made by Litespeed, and certainly looks like a Litespeed build, but I haven't seen an explanation of why they could sell them for £700. Konas and Rocky Mountains were made by TST, which incidentally was at that time a subsidiary of Sandvic but didn't always use Sandvic tubes (TST was a ti tube wholesaler as well as builder, and wholesaled other brands of tube than just Sandvic). Orange and VooDoo were built by A-Pro. I don't know about other brands.
The VooDoo D'Jab is a good example of the the answer to your question, in that it was always made by A-Pro, even in the twentieth century when the Bizango, Wanga and Bokor were made in California in the same factory as the Kona Hot and Ku. It was an excellent facility where Mountain Goats were made and it was still lead by Jeff Lindsay. You won't find better, but they didn't have the facilities to weld the D'Jab, so that was contracted to A-Pro who did.
