It is complicated and has as much work as doing the series.
I use pre-made and free-use sounds, but not always free means that I will find that sound that I just need and that usually comes in the paid libraries.
In the case of my web series, being of horror, I like to make use of strange sounds, laments, noises of tuneful strings, etc. To create an extremely disturbing atmosphere. (apart from electronic music) I like to make the sound with either the Korg VST, which brings some very good sounds that are regulated by the MIDI controller keys. I make other sounds directly with the computer microphone and then process it using echo cameras with the recording software.
Many of these sounds still do not appear in the series, since the design must be according to the scene and until now I have used only the music that I have created to set everything up. But there are some scenes later where those strange sounds go.
For example, in a scene where the character enters a crypt, you would normally be a sound of cellos and violins using Minor scales, but if you switch to strange sounds, detuned strings, guttural sounds of what a spectral creature might be, the environment of the scene changes radically. This is much more effective, by the way, in a large room with the big screen and full of spectators. In the computer that effect drops by at least 50%, so it is hard to achieve that degree of intimacy with the viewer.
I like to use the sound of the storm. Lightning and thunder and the rain begin to fall, as a prelude to the fact that something very bad is going to happen. It is a bit like the use of color: the sound determines not only what is being seen on the screen, but also what is going to happen: anticipation.