The 1 billionth Roblox user is widely reported as Azoo5573. Sources discussing Roblox’s 1 billion user milestone identify that account as the billionth user, and later videos and posts refer to the same username tied to profile ID 1000000000.

About the wording

“1 billionth100 thousandth” looks like two different milestones smashed together, so the clearest reading is the 1 billionth Roblox user. The results I found do not clearly identify a separate “100 thousandth” user in the same way.

Quick note

There’s also a lot of internet lore around this account, including videos and forum posts, but the basic answer is the username Azoo5573.

If you meant the 100,000th Roblox user instead, I can answer that version too.