I need help can someone explain NCT to me

I’ve heard a lot about NCT on TikTok. At first I thought they only had one subgroup NCT U and I thought it was the same as twice and MISAMO but then I keep seeing these other NCT subgroups popping up.

Is NCT one big group or are they just like a collective name for the subgroups. Are the subgroups made up of NCT members or do they have new members in them? Is it similar to the Japanese group? AKB48 where they have 20+ members and some of them do different things.

I’m so confused please help.