Even the Friends writers don't know what Chandler did for a living. While the show was about 20-somethings trying to make their way while living in New York City, it also tackled the professional lives of the main six. Everyone had their respective line of work and Friends did a good job tackling their failures and successes in that regard. However, while it's clear what his friends' jobs were, Chandler's was never really explicitly revealed.
That doesn't mean that the show never showed him working. In fact, he's the only one who consistently held his work for most of Friends' run. Several episodes were set in his office, and he had various interactions with his co-workers. Yet, no one mentioned what exactly he was doing. It also seemed like he's good at it considering that he was offered a promotion and was doing well financially enough to help Joey who struggled as an actor before landing his gig on Days of Our Lives, however, and Chandler was even easily able to afford to get married without much fuss.
Chandler eventually left that mysterious job in Friends season 9 and started a new one at an advertising firm with the help of Monica. It's such a big mystery in the series that even his friends were clueless as to what he did for a living. In season 4, episode 12, "The One With The Embryos," Chandler's job was the deciding question during the famous trivia contest between the girls and the boys who were competing for Monica's apartment. Rachel claimed that he's a "transponster" but that's obviously not it. Seventeen years later, during the reunion episode, Friends: The One Where They Got Back Together on HBO Max, those involved with the show revealed there's no definitive answer to this query simply because not even the writers of the show know what it was. Interestingly, this was confirmed as the cast played in the special's quiz bowl segment where David Schwimmer/Ross once again acted as the proctor as he did in the aforementioned outing.
There were a few pieces of information mentioned throughout Friends that gave an idea of what Chandler's work was. For starters, it's confirmed that he worked in statistical analysis and data reconfiguration. His company also used Moss 865 which Monica's ex-boyfriend Pete Becker invented. Lastly, he also mentioned encoding data which tracks with the idea that he was potentially in IT. This also explains why he's able to afford essentially supporting Joey considering that the industry is known for having high-paying jobs. Despite this, however, Chandler ultimately walked away from it and towards finding his true passion. So while he had to start as an intern and then a junior copywriter who had significantly low wages, he was happy because he genuinely enjoyed it more.
While the Friends reunion didn't canonically show how the Central Perk gang is doing almost two decades since they bid goodbye, Courteney Cox believes that now, Monica and Chandler are still happily married and have kids. No mention about how she thinks his journey in advertising panned out, however. That said, considering that they're still supposedly fine, chances are that Chandler eventually made his way up in the company as he did in his old job.