These are called active present participles. They can work as alternatives for "who was", acting as a kind of adjective to describe what the subject is doing. So,
Мальчик, читающий книгу
is equivalent to
Мальчик, который читает книгу.
It's formed like this:
1. Take the imperfective form of the verb (читать, смотреть)
2. Conjugate into the 3rd person plural (читают, смотрят)
3. Remove the final т (читаю, смотря)
4. Add the ending -щий. This then declines like an adjective, so you can have читающий, смотрящая, etc.
5. If it's a reflexive verb (such as находится), the reflexive suffix (-ся) is "re-attached" after the participle suffix. So we have находящийся, находящаяся.
I hope this helped! Удачи!