Question about transistor on MMIC

For example we're going to design a power amplifier in ADS, we usually do device simulation then schematic simulation and then layout and EM simulations. In the layout phase, how do people place the transistor on the layout? I just don't understand that when we're doing PCB based design, the transistor has a spice model so we only need to simulate the PCB layout and do circuit EM co-simulation, but when we're doing integrated ones, how do people model the transistor in the layout? I know some of the typologies of transistor in MMIC, but do we just draw the materials on the layout and then we can see the device property from EM simulation?

Take this layout for example, how do we simulate the transistor in layout? just draw it and do EM simulation? What about DC property?

EM Simulation cannot involve into Circuit Simulation with SPICE parameters or s-parameters of the devices.
The layout is simulated first without active and passive elements then this is re-simulated with s-parameters in circuit simulation set-up.
You can add SPICE and/or s-parameters of the components in this set-up.SP,AC,DC,HB,TR etc simulations can be done in this schematic set-up
Refer to Co-Simulation of Keysight ADS Manuals..

Note : A new RFPRO Option of ADS 2019 has a capability to simulate s-parameters only with active and passive components and their small signal models.

Thanks a lot. So do you mean first simulate the layout without the transistor, and then simulate the transistor model and the layout EM model together? But how do we consider the connection of the transistor?

Yes, absolutely..
You should place a port for each connection of the transistor ( S D G or C B E etc.) then you will able to connect the transistor to those Ports in schematic view.