Good primer on hardware here;-
https://www.best-microcontroller-projects.com/how-rs232-works.htmlIndividual Amigas vary a bit in terms of maximum speed they can handle. 19200 is usually OK, some will go 38400 even with no fast RAM.
If the Amiga is running a lot of tasks, it might not be able to keep up.
The simplest connection is to swap pins 2 and 3 on the cable between machines, blob 4 and 5 pins on both machines (handshaking pins) and connect pin 7 between both machines (GND). That's on a 25 pin connector found on every Amiga.
The "HyperTerminal" program on Win98 can indeed be used, you have to select which COM port is in use on the PC side. The snag with the PC side is finding a machine which runs Win98 AND has a serial port. These can be 9 pin serial on PCs rather than 25 pin, but they are compatible in terms of voltage levels and timing (just use different pins).
This is for connecting very locally, ie in the same room. If you want to connect over the phone lines instead, to a remote computer, each machine needs to have a modem. And you don't swap the pins going to the modem, it handles the data swapping itself.
Typically a terminal emulator program on a computer (like NComm or Zterm on the Amiga) can also transfer files. The transmitting computer (the sourrce of the files) you point and click at a file using the menus, and the receiving end usually picks that up and starts receiving.
If you type on one keyboard, the text appears at the other end. To see what you have type, you usually have to select "local echo".