Learn Hardcore German: Wir bleiben über diesen Kontakt in der App miteinander in Verbindung. - We stay in touch with each other via this contact in the app.

in Verbindung bleiben sounds a touch more formal/neutral (common in business).
in Kontakt bleiben is very common and slightly more casual. Both are fine here; many speakers would pick in Kontakt bleiben in everyday speech.

Why is it über diesen Kontakt and not something like durch or per?

For communication channels, German most commonly uses über (“via/through” in the sense of a medium): über WhatsApp, über die App, über diesen Kontakt.

per is also correct but feels more formal or written: per E‑Mail, per App.
via is fine, somewhat technical/borrowed.
durch is usually physical “through/because of,” so it’s not the idiomatic choice for a communication channel here.

Which case is diesen Kontakt, and why not diesem Kontakt?

It’s accusative singular masculine. über (in the sense of “via/about”) governs the accusative.

Nominative: dieser Kontakt
Accusative: diesen Kontakt (used here)

Why is it in der App and not in die App?

in is a two-way preposition. Use:

Dative for location/state: in der App (“in the app,” no movement)
Accusative for motion/direction: in die App (“into the app,” movement)

Here it’s a location (the app as the place/medium), so dative: in der App.

What’s the gender of App, and is der App correct?

App is feminine: die App.
Dative singular feminine is der, so in der App is correct.

Do we need miteinander? What does it add?

It’s optional, adding the idea “with each other/mutually.” Without it, Wir bleiben … in Verbindung still means “we’ll stay in touch.”
If you name the counterpart, you’d use mit: Wir bleiben mit dir/Ihnen in Verbindung.
miteinander essentially stands in for “with each other.”

What’s the difference between miteinander, einander, and zusammen?

miteinander = “with each other” (adverb). Works well here: miteinander in Verbindung bleiben.
einander = reciprocal pronoun; used as an object: Wir sehen einander, Wir helfen einander.
zusammen = “together” (doing something jointly). It doesn’t collocate with in Verbindung bleiben for “stay in touch.”

Why is there no article before Verbindung in in Verbindung?

Because in Verbindung (bleiben/stehen) is a fixed idiom. You don’t add an article.
With an article, in einer Verbindung means something different (e.g., “in an association/fraternity/relationship”).

Can I change the order of über diesen Kontakt and in der App?

Yes. Common options:

Wir bleiben über diesen Kontakt in der App …
Wir bleiben in der App über diesen Kontakt … Both are natural. German often orders adverbials roughly Time–Cause–Manner–Place; here, “via this contact” (means/manner) and “in the app” (place) can swap without issue.

Is Wir bleiben verbunden correct, and how does it differ?

Yes. Wir bleiben verbunden is idiomatic and slightly more formal/stylish (“we remain connected”). It’s common in business closings.
It’s a bit less explicit about the channel than in Verbindung/Kontakt bleiben.

What does Kontakt mean here—person or channel?

It can mean:

A contact entry/person (in your app’s contact list),
Or the communication channel/thread with that person.
Here, über diesen Kontakt in der App suggests “via this app contact/thread.” For a person, you can also say Kontaktperson or Ansprechpartner.

Is durch diesen Kontakt wrong? What about via?

durch diesen Kontakt isn’t wrong grammatically, but it’s not the idiomatic choice for a channel. Prefer über/per/via.
via is acceptable and common in tech contexts: via die App / via diesen Kontakt.

Any shorter or more natural alternatives a native might use?

Wir bleiben über die App in Kontakt.
Wir bleiben über diesen App‑Kontakt in Verbindung.
Wir bleiben hier in der App in Kontakt.
More casual: Schreib mir einfach hier in der App.

Pronunciation tips for key words?

über: UE-ber (long “ü” like French “u”), stress on ü.
bleiben: BLIGH-ben (like English “blibe” with bən).
Kontakt: kon-TAKT (stress on the second syllable).
App: like English “app.”
miteinander: mit-ein-AN-der (main stress on an).
Verbindung: fer-BIN-dung (stress on bin).