reckoner
There are two ways, and they are different in one important respect: who decides what is in it.
The way Garmin gives you
You build a strength workout in Garmin Connect, on a phone or in a browser, and send it to the watch. It then walks you through the exercises you chose, in the order you chose, for the sets and reps you entered.1
That works, and it is free with a watch you already own. What it asks of you is everything upstream of it. You pick the movements, you set the loads, you decide when a lift is ready to go up, and you notice when one has stopped moving. The watch is carrying your decisions rather than making any.
The way a coach would do it
A coach does not hand you a fixed program and walk away. They watch what happens to it. A lift that moved well gets more, a lift that stalled gets a change, and a week where you turned up wrecked gets less than the plan said.
What Reckoner sends
Reckoner writes the week and pushes it to the watch over the air, so the day is on the screen before you touch anything: the lift, the load, the reps and the rest. There is nothing to build and nothing to type in, and it works with no signal once it has arrived.
The week after that is written from the week you just did. Every set you log goes back to the engine, and the loads and the rep targets on the next one are its answer to what it read. You are not maintaining a program. You are training, and the program is being maintained from what you actually lifted.
Both routes put a workout on the same watch, and either is better than lifting off memory. The question worth asking is which one you want to be responsible for keeping up to date once the first four weeks are behind you.