Answering the specific question:
would it be physically possible to drive from [Germany to Thailand]
What I mean is that: Are there any roads connecting mainland Europe ... with Thailand
The simple answer is "yes", there are two ways to go:
The normal simple route (blue) is you just drive through Russia then China. (The "high road," if you will.) It's that easy. Hop in your car and turn on the GPS.
Alternately you can take the "low road" shown in red. There are any number of variations and you pass through 10 or so countries.
You pass over no water. It's generally flat.
On the normal (blue) road all roads are sealed and straightforward.
Could it be your question is about The Himalayas?
The question "is Berlin connected to Bangkok by land" is trivially answered by glancing at a globe.
For young people reading, I highly recommend opening "google maps" and if you zoom way out you can get a good understanding of this planet.
However, notice the area marked in pink. If you go there you will find a really, I mean really, big mountainous region of this planet. It's basically not possible to drive through it.
It could be that the sense of your question is basically "can I drive through the Himalayas?". Answer, no, you just go "above" them, ie drive through Russia and China.
Addendum - paperwork
Current political conditions (2023)
As of writing, Russia (the large country at the upper part of this planet which defeated Nazi Germany 80 years ago) is having a war that is apparently disapproved of by some/most political figures in the USA.
Every single web reference I can find states that it's still perfectly possible to travel to Russia (random example)
OP is not a USA citizen so the issue seems irrelevant.
(If you're a USA citizen and reading this, it seems the USA government has put up many pages on the web stating that they "recommend" USA citizens don't travel to Russia currently. However, every serious reference I can find states that travel to Russia (for US citizens) is perfectly possible and normal (random example).
You would have to travel through the regions of "Chaos-stan" as it's sometimes called.
This is a full-on chaotic war zone with irregular and ad-hoc social and political structures.
China paperwork addendum:
It is notoriously a HUGE paperwork fuss to drive in China and/or take your own car in to China.
It's perfectly possible, and people do it every day, it's just a huge, huge, fuss.
Obviously, China has far and away the best roads on the planet.
They are so far ahead, and getting further ahead every month, there isn't even a second place.
(Much as Germany had the best roads on the planet in, say, the 1950s [they are now all falling apart] and the US had OK roads in say the 1970s [they have all fallen apart].)
(Also, of course, obviously, China is far and away the world's biggest car maker and, if I'm not mistaken, the biggest purchasor of cars. It is a completely car-obsessed country, it's the #1 "car nation".)
There are any number of major TV documentaries about the wonders of driving in China, including say
(I encourage you to watch this if you're unfamiliar with the roads there.)
(Note that that documentary is very out of date, roads in China are now another huge leap ahead.)
Actually, in the same show they take a number of foreign cars in to China (as well as driving local cars) so you can also see that process.
The question at hand is about the physical connection. Since, obviously, Germany/Thailand are on the same landmass for the last few million years, answer is "yes".
Hence, I believe the actual sense of the OPs question was perhaps about "The Himalayas". You can not drive through them. You go either above or below them.
Geopolitics issues: the "red" route goes through the planet's current "region of unrest", in short "an anarchic war zone".
addendum sealed roads. It's commonplace to drive across Russia. I understood that it's all sealed, but it's unclear, some of the road may be unsealed. IDK. You can google any number of articles "drive across Russia"