Thanks @Jeffrey. As I mentioned in other comments, we decided to hire seniors to skip the "training" part. This is because we needed start shipping software and build upon existing features and established applications.

Initially, the company thought that they could use a senior iOS developer to build frontend applications using React. The candidate tried his best but it wasn't for him. That proves that being a senior is linked to a specific skill set. You can be a senior in iOS development but a complete beginner in Web Development, Machine Learning, Android Development, etc. This is because you haven't gained the required skills and experience in those areas yet.

The company truly tried to help those engineers. But they quickly determined that it was counter-productive and the process was going against the initial thought of hiring a senior from the start.

Instead, I suggested hiring junior and mid-level engineers and training them to reach the senior role. That meant they needed to give up the idea that they could hire someone who could start shipping new features the first month they got hired and pushed deadlines a bit further to account for the "training" part.