How is Trilogy Hub different from MTA:SA?
The biggest difference is the version of the game. MTA:SA runs on the original GTA San Andreas. Trilogy Hub runs on GTA: San Andreas Definitive Edition, the Unreal Engine 4 remaster.

Friends and chat in the launcher
MTA:SA has a server browser with favourites and history. Trilogy Hub has a server list in the launcher, next to your account, a friends list with their status and chat, so you arrange a game in the same window you start it from.
Can I move from MTA without relearning?
Yes. The Trilogy Hub API is written in MTA style: resources with meta.xml, the same function and event names, the same handling of bad arguments. A ready-made MTA resource is worth reviewing, because not every function has a counterpart yet, but the way of thinking carries over directly.
See the page for creators and the Lua documentation.
Questions
How is Trilogy Hub different from MTA:SA?
They are two separate projects for different versions of the game. MTA:SA runs on the original GTA San Andreas, while Trilogy Hub runs on Definitive Edition (Unreal Engine 4). Scripting works in a similar way, and the Trilogy Hub launcher also gives you an account, friends and chat.
Will my MTA:SA scripts work?
Partly. The Trilogy Hub API uses a similar model: resources with meta.xml, events, exports and the same names for many functions. A ready-made resource needs reviewing, because not every function has a counterpart.