STP - BPDUs, VLAN IDs, and PVST+

When deploying PVST+ as the Spanning Tree Protocol (STP) on a switch, a separate STP instance is created for each VLAN. That means that BPDUs are also generated on a per-VLAN basis.

So how do switches know which BPDUs are for which VLAN?

BPDUs do not include the VLAN ID in their headers, but they are are sent inside VLAN-tagged Ethernet frames. The VLAN ID is included in the Ethernet frame, not in the BPDU itself. When a switch receives a BPDU, it determines the associated VLAN by examining the VLAN ID in the Ethernet frame.

https://networklessons.com/spanning-tree/per-vlan-spanning-tree-pvst/