Access to the engine compartment from below

I'd say you're likely to break one or more fasteners. The Gen 3 used these plastic fasteners on the oil filter access panel, and I've never been able to keep them from breaking and/or tearing the plastic panel itself. Gladly, Toyota used screws on the Gen 4 oil filter access!

 

I would expect there would be runner plugs through the firewall from the engine compartment to inside the car.
@Elektroingenieur may know. I know @sfv41901 has worked on Gen 4 and Prime should be similar.

 

I use one of these the most, when removing the plastic fasteners (on our 3rd gen):



A paint can opener with hooked tip. It's relatively easy to hook under the cap and pry up. I also occasionally use a slim bladed screw driver, and once in a while a traditional pry tool, the kind that cup around both sides of the plastic fastener collar:



I've done maybe 14 (DIY) six monthly oil changes on our 3rd gen, taking the engine under panel completely off each time, and at least a half dozen more instances I've taken it off. I've got a few spares of the plastic fasteners, and while I can't really recall, I believe I've had one break, at most.

The main issue: grit can get into the internals, which can cause them to jam when you try to pry up the central post. If this happens just take it real easy, pry both sides of the center cap, use caution.

With my 6 monthly removal, I always wash the fasteners out with hot/soapy water, basically returning them to new condition. This seems to me the main factor in maintaining them.

With your fourth gen, there may be more than one fastener style. Assuming that, I would see if you can get a printout with the parts numbers, which go where.

 

Perhaps @Elektroingenieur could share a graphic of the fastener placement on the Prime, if there is more than one fastener type.

 

I would, but I haven’t found a detailed illustration of the underside of the Prius PHV. The Repair Manual just says, “Remove the 10 clips, 4 bolts and No. 1 engine under cover assembly,” without showing the fastener locations. For reinstallation, the four bolts are torqued to 7.5 N·m {76 kgf·cm, 66 in·lbf}.

In parts catalog Figure 51-02, Suspension Crossmember & Under Cover, the second illustration (enlarged view) shows the No. 1 engine under cover assembly (51410-47070) and its ten plastic clips (90467-07220) and four bolts (90119-06A13). These Toyota part numbers are common to RHD and LHD vehicles.

If you’re careful, you can sometimes remove and reinstall the plastic clips without damage. I’d suggest buying a few spares, though, especially if you might need to remove the cover assembly more than once.

If @zigzag is considering a new connection to the auxiliary (12-volt) battery, he might also be interested in my posting about the battery state sensor; I believe the general arrangement on the Prius PHV is similar, though some of the reference designations are different.

 

I took a short look a week or two ago with the car on the ground, how to drop the panel and I noticed
( mostly felt around ) both bolts and pins/clips.
From experience, the longer the clips stay in the car the easier it is the break them when trying to remove them.
If the pins exercised and lubed before replacing they are easier to remove the next time,
no matter what condition they are in when replaced.
I'm still not sure which lube is best, but any oil or silicon based lube make removing them easier than reinstalling the clips dry
Push Pin Pliers
I got one of these for $10 harbor freight, but only because I had to remove 40 or so that had been in for years
and I wouldn't recommend them for newer clips. But they will get pesky ones out even if they have to be broken out.