The primary factors for how much data a program can handle are appropriate data structures and algorithms. When it comes to presenting this data on-screen, that’s a concern, too. A too-simplistic GUI could choke on a surprisingly small amount of data.
Data structures are stuff like lists, hashmaps and eventually even databases.
Algorithms are what processes this data. A super common use case is text search.
A GUI must not create GUI elements for too much data at the same time, but ideally just enough for what’s currently visible, plus some other concerns.
If software uses inadequate data structures and algorithms, it cannot optimally utilize the computing power or memory available. One striking example is the ZIP archive support in Windows Explorer. It used to read archives one byte at a time, resulting in ridiculous performance even on the fastest of computers.
OneTab is proprietary software. You’d need to reverse-engineer it to get answers.