To unlock the Alphabet Legacy trophy in The Sims 4, you will need to play as a single household for 26 generations.
The quickest way to unlock this trophy is to first build up some money, either by playing naturally and unlocking this trophy as one of the final trophies you unlock, or by managing worlds, and having people move into your house and selling the remaining lot furniture.
Once you have money, evict everyone in your household so that you only have one adult remaining. Adopt a child which cost $1,000, and then force them to age by cooking a cake and having the child blow out the candles. Do that twice, and they will age from Child > Young Adult > Adult.
You can then evict the original adult if you wish (or leave it until you have seven adults to evict which might be a bit quicker, but potentially more confusing).
Have the newly aged adulty adopt a child, force them to age up to adult with a cake again, and rinse and repeat until you have worked through 26 generations.