Answer: Sitting is a milestone and this is a case of different strokes for different forks.
Some kids are aided, as in trained while others fall into it automatically.
The normal sequence of baby development follows this order: rolling, pivoting, pushing backwards, and lurching forward, moving forward on tummies, leading to tummy crawling then creeping. It is at approximately the same time that a baby gets him/herself into the creeping position on her hands and knees that she will push him/herself back to sit. So, developmentally, babies don’t need to sit until they are just about to creep/crawl”.
At 4 months, a baby typically can hold his/her head steady without support, and at 6 months, he/she begins to sit with a little help. At 9 months he/she sits well without support and gets in and out of a sitting position but may require help. At 12 months, he/she gets into the sitting position without help.
You don’t do training due to risk to damage on the spinal cord and widening of the pelvic bone.
Read more here: https://elimuzazi.com/2022/11/26/should-i-train-my-baby-to-sit/