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Leaving aside the "why on earth would you want to do that?" question; you could configure a bridge on OPNsense, with multiple NICs as members, and disable filtering on the bridge, which would effectively implement a software ethernet switch. https://docs.opnsense.org/manual/how-tos/lan_bridge.html

This helps, thanks. However, I found that assigning Bridge to LAN won't make it a switch. What I did was assigned ports to OPTx and bridged these OPTs and assigned one WAN to connect to the company network. In the mini computers, there are 5 Ethernet ports, I assigned 4 ports to OPT and one to WAN. I plugged WAN to company network and the rest can connect to other devices. this way I can access the Web GUI to manage OPNsense.

I guess, they can all be assigned to OPTs. I mean all the 5 ports, but you will have no management access to the Web GUI, but using a console port to manage it in text mode, but most people don't fit in the scenario.

This is a fabulous way to get fired, by using a switch on someone else's network. :D

Also, you may want to google what a switch really is, before you try to make one. :D

three are switches, which connect to computers in the office. When I add one more simple switch, what is the problem going to be? I wanna build a switch not a router, I don't think that is going to mess anything up as long as I'm allowed to have multiple devices (computers).

At work, I want to make a simple switch by using OPNsense on these mini computers with LAN ports on. There are many, but not used at the company.

It is intended to be a network switch and a really simple one, like the one you can buy for $20. I know it is not power efficient, but that is okay. At least, these mini computers can be useful again.

I'm a newbie to networking. I already install OPNsense on one of these mini computers but don't know how to set it up to be a switch.
I tried but failed. I don't need it to run firewall, dhcp, and routing, just need it to be a cheap switch, which I can connect a couple of devices to and use the internet.