FRA airport: Arrive to A, depart. from Z, do I need to go through border?

You will go through passport control. (I know a lot of people call this "customs" but that is something separate and not of concern to you as you are just transiting.) I am not sure where Visa checks occur, but I would assume at passport control.
How long is your layover? I transited through FRA last week (went from A to Z, both are in Terminal 1), and I barely made it with less than an hour and a half. I was on a smaller plane initially and had to take a bus to the terminal, which was the vast majority of the delay. The buses were at a standstill.
I spoke with a flight attendant about it and he said FRA is a disaster, and they are about to embark on a major renovation that will take years. So, soon FRA will be one of the airports to avoid if possible. If you have about three hours, you should have ample time to take a bathroom break, purchase water, etc.

@valadelphia, thanks a lot! The layover is 4h 50mins, seems to be enough even if the move is a nightmare!

Do you know by any chance if it’s possible to go through A to Z without a visa? The papers of German borders say that only certain countries would need a transit visa, but I’m not sure…

This depends on your nationality, where you are going, and for how long--google "Kazakstan and Visa"?
https://travel.state.gov/content/travel/en/international-travel/International-Travel-Country-Information-Pages/Kazakhstan.html

Hi, if you're flying FROM OPO (Portugal right? I looked it up) to ALA through FRA, you shouldn't need a transit visa if you're a EU / or US citizen. What is your citizenship? Have you checked your outgoing flight operator from Portugal to see what they say is required for transiting? My guess is nothing, but I would double check! 4+ hours is heaps of time even in FRA, where I've transited a few times in past 6 months, yes lots of trips by bus but easily made flight within 2 hours.

Given that Portugal and Germany are both in Schengen the OP would not need a transit visa. This is a "domestic" flight, and the OP is already in the Schengen Area....

I spoke with a flight attendant about it and he said FRA is a
disaster, and they are about to embark on a major renovation that will
take years. So, soon FRA will be one of the airports to avoid if
possible.

They are building a complete new terminal in FRA. Should become operational in 2026. I assume they will not start the renovation at the old terminal until the new one is operational.

Yep, 99% of travelers would not need a transit visa, however the following actually DO when transiting in Germany. I was surprised to see India on the list.

Most travelers do not need a transit visa to pass through a German airport if they stay within the international transit area and don't go through immigration. However, some countries require nationals to obtain an airport transit visa before booking a flight through Germany. These countries include: Afghanistan, Bangladesh, Democratic Republic of the Congo, Cuba, Eritrea, Ethiopia, Ghana, India, and Iran.

Hey OP, Agreed, FRA isn't the easiest airport to navigate and there can be delays, but I've not missed a flight with 1.5 hours layover traveling through there multiple times a year, so you should be fine with a bit of time.

I think some answers are confusing the situation.

The OP is starting in Porto, PT, flying to Frankfurt, then on to Almaty, Kazakhstan.

The OP does not mention what nationality they are, but neither EU member countries, or US passport holders need a visa for Kazakhstan.

However, one travelling the route laid out would need to do exit immigration from the Schengen zone, moving from the Schengen terminal to the international terminal in Frankfurt. If a visa is required for the OP (not a transit visa), then yes, they would need to show that as part of their Schengen exit process.

The "Transit Zone" at T1 in FRA is the Z Concourse, parts of the B Concourse, and the C Concourse, what is often called "out-of-Schengen". It is for flights to/from non-Schengen countries. The transit zone is connected by the Skyline, which it has dedicated cars with doors that only open to the transit zone side in stations in the Z, B, and C Concourses (actually, the C Concourse has only transit zone gates. The doors of the "public zone" [Schengen side] cars don't even open at C).

"Exit immigration" is actually called "emigration" and is the passport check you go through when leaving Schengen.

The OP is coming from Portugal, a Schengen country, so presumably she will have either a US or EU passport or a visa for Schengen. She would not need a transit visa. A30 is not in the transit zone, Z50 is.

The A/Z building has two wings. The new one is parallel to the runways and has gates, both A and Z gates with numbers 50 and above. The original wing, which juts out at 45° to the new wing, has gates with numbers up to 40. A30 is at the far end of the wing, in a lower section for regional jets, A gates only, which is also parallel to the runways.

From A30, it's a long walk, almost 2000 ft, to the emigration counter at the "hinge" between the two wings, across from gate A11. After going through emigration, there are escalators up one level to the Z Concourse, where gate Z50 will be the first gate in the new wing, almost another ¼ mile. Overall, the walk about 6/10th mile (1 km).

Here is the map.