Which cable should I use for target disk mode?

To connect ThunderBolt-3 devices to your older ThunderBolt-2 devices, you can use the Apple ThunderBolt-3 <-> Thunderbolt-2 adapter, plus a genuine ThunderBolt--2 cable on that side. This adapter does not supply power, so each device must be self-powered (e.g., does not work for Bus-powered drives.) This adapter does not support displays, except genuine ThunderBolt displays.

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m?fnode=ae621829d455c3cdf2156d058578422c16daf5827371d14f1e520a7baeb3b1f36d18669ebbe4744de9e80fdd79bc3209d13db07f32920bd45e88e7b3d58225a6a64de7ed368cf8979d4f5f39eec0e284b35b6f6b33a1e4314989fe88a16c0667

BEFORE the introduction of ThunderBolt-3 USB-C you could generally just mate the correct connector shapes and it would all work. ThunderBolt-3 and its USB-C connector throws all that out the window, and now you need to look carefully at the symbol on each end of the cable each time, and often understand even more about the devices involved.

ThunderBolt-3 is an uncommitted computer port that can do almost anything, with the right cables/adapters. The first cable/adapter you attach specializes the port to one particular purpose, usually a more narrow subset of all it could potentially do.

The printed specs for each machine tell you exactly what the ports are and their protocols.

Macbook Pro 13" 2016

Charging and Expansion

Two Thunderbolt 3 (USB-C) ports with support for:

• Charging
• DisplayPort
• Thunderbolt (up to 40 Gbps)
• USB 3.1 Gen 2 (up to 10 Gbps

"DisplayPort" is a specific type of port and has it's own specific attributes

"Thunderbolt" (up to 40 Gbps)

"USB 3.1 Gen 2" (up to 10 Gbps)

The above are specific port identifiers and are given with the max speed of data transmission for each.

Thunderbolt 2, Thunderbolt 3 (USB -C). Thunderbolt 4, USB-A and other similar all are specific names and have specific connector configurations and data speed limits.

Search with Google or your favorite search engine on the protocols and connection/connector types and you'll find literally hundreds of appropriate cables to choose from.

The cables you need is entire determined by the ports you have available one all machines. You'll need to buy a cable for each unique setup, or maybe a single cable for the prime Mac and adapters to match the others. A simple inspection of the ports will tell you what you need. Maybe also consult the web on each different model's specifications as some dissimilar ports look alike.

Amazon, Newegg, Apple and many other site market all sorts of cable and adapters.

Generally yes, but you have to take it within context. A writer may use "Thunderbolt" as a general class and then expand on that in the body of the article or whatever. Thunderbolt 2 is essentially dead. The connectors look like DisplayPort cables except DisplayPort connectors have a tiny monitor pic on them. Thunderbolt 2 lack the monitor graphic and physically the connectors will fit both female ports. The difference is the DisplayPorts do not carry data, Thunderbolt 2 does carry data. The devil is indeed in the details.

This IS a can of worms, but if you're going to undertake such a project, you owe it to yourself to understand the ports, cables and protocols. And if by chance you buy inappropriate cables, you've just joined a well populated worldwide group. That's why I always buy cables from a vendor with generous return policies.

There are site on the web that try and explain it all. Those are far better places to go than a user to user forum. The subject is just too much for such as here if your goal is to learn something. And as a former HS science teacher now retired, it's always a worthy goal to learn rather than memorize.