Connecting 2 Offices to the same Network

Hello everyone,

so I post this in order to get some guidance and knowledge about my current problem. I didn't feel like this post belonged in any other subreddit, but if you think so, please advise me on where to post this question.

So basically I have no networking experience or whatsoever. My task is to connect 2 offices to the same network. Right now they have 2 seperate networks. So what should be done is connecting the office B, to the network of office A. The offices are in the same building, on the same floor, right across each other, so the distance allows it for them to be connected via cable.

The office A Setup:

• UniFi Switch 48
• Ubiquity POE
• Ubiquity Cloud Key
• Ubiquity Security Gateway
• Cisco Modem EPC 3925 (from our ISP)

The office B setup:

• Cisco SG102 - 24 Switch
• Cisco Modem EPC 3925 (from our ISP)

So, my question is, how do I do this? I have this naive and stupid tought that it should be as simple as plugging the cable at one end (Office A) to the other end (Office B) and than they should be on the same network than. So, what do I need to do in order to make this possible? Where should be the cable plugged in? Do I have to do some changes on the network before or after connecting the 2 offices? What about the existing hardware infrastracture? Can they be connected to each other with existing hardware setup at all?

As you can see I am completely clueless with some of the basics and am definetely not even an amatuer. I am however eager to learn and I do not expect solution from you, I just need someone to point me at least in a right direction where to look for the solution. I also understand if this post gets removed as this is a subreddit for networking professionals, just didnt know where to post else. Thank you!