do I have to hook up the old and new computer or will it transfer side by side?

When you buy a new Mac the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes  and follow the on-screen prompts and it's done! It is THAT simple.

If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts. 

The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

Migration Assistant is a program provided by Apple for transferring your files from your old computer to your new computer. It is very easy to use. Be sure to read and follow its instructions. Basically, the process is as follows.

You first begin by starting the process of setting up your new computer. You will be asked if you wish to transfer files. Respond YES and follow the instructions you will be given.

·      You will be told to turn both computers on and activate Migration Assistant in the old computer.

·      The computers should be connected with a cable to facilitate the transfer. Wi-Fi is not suited for a transfer of so much data and corruption often results. If you must use it, make sure the two computers are located close together.

·      You will be given several options of what to transfer. You will likely want to transfer your settings and preferences.

·      You can transfer all or none of your files. You do not have an option to select certain files to transfer. If you wish to do that, then do so manually. You can transfer all or none of your installed apps. Some people elect to download fresh installations to the new computer instead.

·      The old computer's operating system will not be transferred. No data from the old computer is erased.

·      Allow for up to an hour for the transfer process to complete once you start it.

·      It should set up the new computer the same as the old one.

·      When that is successful, you can log into the new Mac with the old User Account name and password and delete the brand-new user or leave it in place as a second admin account.

Enjoy your new Mac.

I hope this is helpful. Good luck.

There must be a connection of some sort; Wi-Fi is often not workable because it's just too slow in many cases and prone to failure.

I prefer to attach the Time Machine drive directly to the new Mac via USB cable. If you have no TM backup, it's easy enough to make one: Back up your Mac with Time Machine - Apple Support

Transfer to a new Mac with Migration Assistant - Apple Support