I have two unifi access points inside and another two outside due to brick exterior walls that blocks most wifi. It seamlessly switches, to the point I can be on a video call on my phone and wander around without any interruptions.

Controllers that support roaming and handoff.

You don't even need a controller for roaming/handoff (although it increases efficiency).

Any standards-compliant 802.11 access point will support basic roaming, even across different vendors, since it can be entirely client/station-driven.

In particular, 802.11r isn't that hard to support.

Hope you weren't downloading a file

It’s more than fine. We have no cell signal (so WiFi calling), 5 UniFi APs. You can walk around the place and you wouldn’t even notice a transition.

If downloading a file makes your WiFi calls bad, look into bufferbloat and CAKE.

It’s seamless