Williamson only made his Glasgow debut last November but his impressive club form won him a call-up to Scotland's squad for the tour of the Americas.
Williamson made his debut when he started against Canada before coming off the bench against the USA and Chile. He'll start again in Saturday's meeting with Uruguay in Montevideo.
"It's been pretty surreal," said Williamson. "From making my Glasgow debut earlier this season to where I am now, it's been quite a journey for me. I've loved all of it so far.
"I didn't see any of this coming to be honest, I'd been quite big on just taking it one week at a time because you never really know what's going to happen. I'd have laughed at you if you told me at the start of the season everything that was going to happen."
Williamson says his development has been helped by playing alongside Richie Gray and Scott Cummings - both experienced international locks - at club level.
"It's brilliant to have both of them," he said. "Richie's done pretty much everything in the game and his knowledge and the calmness he brings really helps me.
"I learn a lot of detail off Scott, he's been great with me, looking at my performances and helping me kick on. Both of them together, I couldn't really ask for much better role models."