Getting close pictures of small things (usually insects) is difficult. When there’s something small that I want to take a picture of up close I use my 400 mm lens with extension tubes (36 and 20, 56 mm total). Extension tubes let you focus closer to the subject than normal. However they don’t increase zoom and they decrease the amount of light coming to the sensor. For long distance shots I use teleconverters (1.4x and 2x, 2.8x total). This turns the 400 mm lens into an 1120 mm equivalent. However, these also use a lot of light and slightly decrease image quality. When I want close focusing and zoom I can use both. This severely decreases the amount of light. You need a very bright light in addition to a tripod (with a very good head, you’ll see why soon). These are longer though so they change my 8 inch lens + camera combo (12 in zoomed) to a 13 inch (17 zoomed). This makes it difficult to carry around attached. The problem with this is unless you have a camera mount attached to the lens you have a lot of leverage on the tripod head. Sometimes you need to hold it up (which decreases quality) to prevent it from pulling the head down (or even the entire tripod if a leg isn’t in front of the lens). If you use the shutter button it can shake the photo causing it to be blurry (especially at high focal lengths). A handheld shutter release is necessary to keep good quality. I didn’t have one so I set it on touch-activated shutter instead (since it reduces vibrations compared to pressing the shutter).
tl;dr you can use both extension tubes and teleconverters to get closer to a subject and increase zoom. However you need a lot of light and a tripod (with a strong head).
Here are some shots I took of a frog (~1" long) this way. I used every light nearby, a tripod, and a ring light (which I held to prevent glare). For the equipment I had a Canon EOS R, 18-400mm tamron lens, EF to EF-R adapter (to use the lens on a mirrorless camera), 36 mm & 20 mm extension tubes, and a 1.4x and 2x teleconverter. I took 4 frames but only 2 turned out good.
