New immigration rules for the UK are being enforced starting February 2026. This means citizens of 85 additional countries (including the USA and European countries) will need an Electronic Travel Authorisation (ETA) to visit, even for tourism. Luckily, you can apply (and pay) digitally. Unfortunately, the government very much would prefer you to do so using the official app.
TLDR: you don’t need an app, you can apply online, but dark patterns are used to try and guide you towards using the app.