You will want to use Hackers Keyboard (available on playstore as well as on F-Droid) for that, which was explicitly written for such purpose:
Hacker's Keyboard (source: Google Play; click image for larger variant)
That would be the most convenient approach – though the app hasn't been updated lately. But if you don't want that, just take a look at the corresponding Termux help page (or the matching Termux Wiki page): Termux utilizes the volume keys for "special combos". You can also have the most common keys shown on the screen to be "tap ready":
The docu writes: To enable the extra keys view you have to long tap on the keyboard button in the left drawer menu. You can also press Volume Up+Q.
Or, as Death Mask Salesman stated in a now deleted comment: "Volume Down equals Control, while Volume Up is the same as Alt."