Where to find intergrations?

Hi, I installed HASS yesterday and this is my first post so forgive if I’m asking bullsh*t. I want to tie my brand new HASS toy with my WAGO PLC that I’ve been using for years. I wanted to use modbus switch but I’m lacking to set coils under one address and to be able to read back status from other address. I saw integration source and it seems easy to add but… where do I find the code of that integration?
I have several containers running:

CONTAINER ID   IMAGE                                             COMMAND                  CREATED       STATUS          PORTS                  NAMES
44cd83d09c22   ghcr.io/hassio-addons/vscode/amd64:3.0.1          "/init"                  7 hours ago   Up 7 hours                             addon_a0d7b954_vscode
8ab6b3b472fa   homeassistant/amd64-hassio-multicast:3            "/init"                  8 hours ago   Up 8 hours                             hassio_multicast
a7ecaf9878d7   homeassistant/amd64-hassio-cli:2020.02.0          "/init /bin/bash -c …"   8 hours ago   Up 8 hours                             hassio_cli
a104f0dc53d5   homeassistant/amd64-hassio-audio:17               "/init"                  9 hours ago   Up 8 hours                             hassio_audio
41c97b85b2af   homeassistant/amd64-hassio-dns:2021.01.0          "/init"                  9 hours ago   Up 9 hours                             hassio_dns
57448bfb3f7f   homeassistant/qemux86-64-homeassistant:2021.2.1   "/init"                  9 hours ago   Up 29 minutes                          homeassistant
2991fbdd64dd   homeassistant/amd64-hassio-observer:2020.10.1     "/init"                  9 hours ago   Up 9 hours      0.0.0.0:4357->80/tcp   hassio_observer
26e02da72f03   homeassistant/amd64-hassio-supervisor             "/init"                  9 hours ago   Up 9 hours                             hassio_supervisor

In which do I look for integrations? I think I’ll better start from editing built in one then trying to duplicate it overriding a built in one.

the integration is built in. You need to configure it in configuration.yaml. see https://www.home-assistant.io/integrations/modbus/

I know that but I want to edit it, not to configure.

What do you mean by edit? Do you want to write/change its code?

If yes. then you can start from here how things work

The code is in the homeassistant container, the one starting with qemux86. But there is no point in editing that as it’ll be wiped when you update home assistant.

Probably the easiest way is to make your changes and use it as a custom component.