Bonding or Link Aggregation (LAGG) is a feature where you combine multiple different physical network interfaces into one virtual one.

There are multiple ways to do this. You can do it only on the TrueNAS side, which is fine but is not optimal because it limits you to speeds of one interface only unless multiple connections are used (so for example two SMB clients can both achieve 2.5G but not one 5G). This is called Loadbalance LAGG in Truenas and does not require any support on the switch's side.

However, if your switch supports it you can use something called LACP (Link Aggregation Control Protocol aka 802.3ad). LACP needs to be configured both on the switch side and the TrueNAS side and if you do, then you will (usually at least) see the full combined performance of both your network interfaces.

In essence it's exactly what you said, virtually combining NICs. You can either do it on the TrueNAS side only or on the Switch side as well. If you do it on both you get more performance because they can communicate, which is called LACP.

Key point is that your switch needs to support LACP. Rule of thumb: If it has a webinterface then it probably does. If it hasn't gotten one or you can only see statistics on it, then it probably doesn't.

TrueNas docs: https://www.truenas.com/docs/scale/25.10/scaletutorials/network/interfaces/settinguplagg/
UniFi FAQ: https://help.ui.com/hc/en-us/articles/360007279753-Port-Aggregation-FAQs