I had two in the 90s and one in the 2000s and used the same techniques. I am a recent grandma, the difference is striking, until the point the sleep consultant was called in and then it was very similar.
So first child I found around three months, was not sleeping as well as I wanted, would not go down to sleep without a lot of rocking etc, not practical, especially as I fell pg with number two around that time.
Sleep trained number 1 (ds) by letting him cry to sleep. Took one week, actually more like a few days. Cried altogether for 1.5 hours the first night and then 1 the second, and it reduced significantly the following nights.
With number 2 (did) from birth I had a clearer structure for her sleep. After the first week or when sleeps were three a day (bottle fed so easier to do ime) I always put her in a different room, so upstairs, in a darkish room. So there was a distinct ..time to sleep…and not using the living room etc. Although, noise was kept the same, I wanted them to sleep in an environment with noises etc.
When at home they went down for their naps, generally around the same time of day, but when very young babies I also relied on sleep cues.
There were no sound machines, sleep windows, sling sleeping or even taking them out in a pram/car to get them to sleep. Because dd was put down awake from birth, she learnt to self soothe. I know the studies would say that’s not possible from a young age, but it worked with all three of my children. They all slept a dream really!