Beim Routenbau ändert sich die Wandneigung, wenn überhaupt, nur wenig.

Beim is a contraction of bei dem.

• bei = during / in / at / while doing
• dem = dative form of das

So:

• beim Routenbau = bei dem Routenbau = during route setting / when building a route

In German, these contractions are very common and usually preferred in normal usage:

• am = an dem
• im = in dem
• vom = von dem
• zum = zu dem
• beim = bei dem

Here, beim Routenbau is a standard way to mean while setting routes or during route construction.

Routenbau is a noun, formed from:

• Route = route
• Bau = building / construction

German often turns actions into compact nouns like this, especially in technical or professional contexts.

So:

• Routenbau = route construction / route setting

In climbing, this refers to the process of creating a climbing route.

Using the noun after beim is natural, because bei is followed by a noun phrase here. Compare:

• beim Routenbau = during route setting
• beim Kochen = while cooking
• beim Lernen = while studying

Because the preposition bei always takes the dative.

Since beim = bei dem, the noun that follows belongs to a dative phrase:

• bei dem Routenbau
• shortened to beim Routenbau

You do not see a separate article directly before Routenbau because it is built into beim.

German main clauses follow the V2 rule: the finite verb comes in the second position.

Here the first position is occupied by the whole phrase:

• Beim Routenbau

So the finite verb must come next:

• Beim Routenbau ändert sich die Wandneigung ...

Structure:

1. Beim Routenbau
2. ändert
3. sich die Wandneigung, wenn überhaupt, nur wenig

This is very normal in German. The first element does not have to be the subject.

Compare:

• Die Wandneigung ändert sich nur wenig.
• Beim Routenbau ändert sich die Wandneigung nur wenig.

Both are correct; the second just puts time/context first.

Yes, sich ändern is a very common German verb meaning to change.

In many contexts, German uses the reflexive form where English simply says change:

• Die Situation ändert sich. = The situation changes.
• Das Wetter ändert sich. = The weather changes.

So here:

• die Wandneigung ändert sich = the wall angle/inclination changes

This does not mean the wall is changing something else. It means the wall angle itself changes.

A useful contrast:

• etwas ändern = to change something
  
  • Sie ändern den Plan. = They change the plan.
• sich ändern = to change
  
  • Der Plan ändert sich. = The plan changes.

The subject is die Wandneigung.

You can tell because it is the thing that is doing the changing:

• die Wandneigung ändert sich

So the core clause is:

• Die Wandneigung ändert sich nur wenig.

Everything else adds context:

• Beim Routenbau = during route setting
• wenn überhaupt = if at all

Wandneigung is a compound noun:

• Wand = wall
• Neigung = inclination / tilt / angle

So Wandneigung means something like:

• wall angle
• wall inclination
• steepness/tilt of the wall

In a climbing context, it refers to whether the wall is more vertical, slabby, overhanging, etc.

Wenn überhaupt means:

• if at all

It softens or limits the statement even more. It suggests that maybe there is no change at all.

So:

• ändert sich ... nur wenig = changes only a little
• ändert sich, wenn überhaupt, nur wenig = changes only a little, if at all

It is a parenthetical phrase, so it is set off by commas.

Because it is being used as a comment phrase inserted into the main statement.

The basic sentence is:

• Beim Routenbau ändert sich die Wandneigung nur wenig.

Then wenn überhaupt is inserted to weaken the claim further:

• Beim Routenbau ändert sich die Wandneigung, wenn überhaupt, nur wenig.

This is similar to English:

• The wall angle changes, if at all, only slightly.

German often places such parenthetical expressions in the middle of a sentence and marks them with commas.

Wenig means little / not much.

Adding nur makes it more explicit:

• wenig = little
• nur wenig = only a little

So nur wenig emphasizes the small degree of change.

Compare:

• Die Temperatur sinkt wenig. = The temperature drops little.
• Die Temperatur sinkt nur wenig. = The temperature drops only a little.

In this sentence, nur wenig modifies ändert sich and describes how much the wall angle changes.

That word order is not impossible in all contexts, but ändert sich die Wandneigung is the more natural order here.

In the middle field of a German sentence, pronouns like sich often come early, especially unstressed ones.

So:

• ändert sich die Wandneigung = natural
• ändert die Wandneigung sich = much less natural here

German often places short pronouns before fuller noun phrases:

• Dann hat sich das Problem gelöst.
• Heute verändert sich das Klima spürbar.

Yes, that is a very good translation.

Other natural versions include:

• During route setting, the wall inclination changes very little, if at all.
• When routes are being set, the wall angle changes only slightly, if at all.

The exact English wording depends on style, but the main idea is:

• the context is route setting
• the subject is the wall angle
• the change is minimal or possibly nonexistent

In this sentence, very likely yes.

Outside of context, Route can mean many things, such as a travel route or path. But together with Wandneigung (wall angle), the sentence strongly points to a climbing context.

So Routenbau here most likely means:

• setting climbing routes
• creating a route on a climbing wall

Because wenn überhaupt is inserted as a parenthetical expression.

German uses commas to mark such inserted comments, especially when they interrupt the normal flow of the sentence.

Without the insertion, the sentence is:

• Beim Routenbau ändert sich die Wandneigung nur wenig.

With the insertion:

• Beim Routenbau ändert sich die Wandneigung, wenn überhaupt, nur wenig.

The commas show that wenn überhaupt is extra commentary, not part of the core clause structure.

Yes. That is a very useful way to understand it.

German often uses a preposition plus a noun where English might prefer a clause with when or while.

So:

• beim Routenbau = during route setting
• beim Essen = while eating
• beim Arbeiten = while working

A noun phrase in German can often correspond to an -ing phrase or a when/while clause in English.