If you do not want to pay for two seperate transport rides (350 HUF each ride/ticket), you can buy a Transfer Ticket (530 HUF) valid for two rides between metro trains, buses, trams and so on.
Your first trip is valid for 100 minutes and your second trip (the transfer) is valid for 80 minutes, but your transfer must be taken within the first 100 minutes. See the BKK Transport T&Cs.
Below you will find a basic, small, review of some apps you should ideally use when living in Budapest and other parts of Hungary. Although some are tourist related, they also contain useful information for those living in Hungary too. Many are developed for devices that use either the Android, Amazon Fire, Apple or Windows operating system, but not for all operating systems/devices; which is "normal" these days.
NOTE - Although the apps below were tested on either a BUSH Android Kit Kat mobile phone, Huawei PLite 8 Android Nougat mobile phone, iPhone 4, iPhone 5, iPhone 6, Alba Android Lollipop tablet, Windows 10 computer or Amazon Fire 5 tablet or a combination of those devices, they might also work on other devices. Hence why you should visit each app developer's website for details on their device requirements.
NOTE WELL - These reviews are purposely not long-winded and purposely do not detail every feature of an app. The reviews are more of an 'overall observation' of how well suited they are, and what they have to offer, for someone living in Hungary. Meaning: No waffle, just straight to the point app PROS and CONS. As the apps are NOT listed in any 'good/bad' order simply read each app review, try the app if need be and then make your own judgement(s) about that particular app.
Although physical books and maps for example still have their place in society, even though many of the major ones have been computerised already, apps have become more interactive than readable books and maps. With apps being pocket-size, more affordable and constantly updated with new features - such as search, up-to-date timetables, history and bookmarks - it makes sense to invest in a few apps prior to you living in and/or travelling around Hungary.
Although many app websites state their app/software works on multiple devices and operating systems, you might find dead links to their associated online app stores. This could be because they no longer sell on those stores and/or for those devices, for whatever reason(s), whereby their own website has not been updated accordingly. The copyright on their website might also state 2014 for example, with contact details missing. These are signs, in my eyes, of lack of support for the app, website and/or you (the potential customer).
An app working on one device but not on another is not really a concern for me. Mainly due to the fact that many devices are affordable these days, new or second-hand, whereby if you really want an app, you will buy or borrow a device for it.
NOTE - On some apps, especially those requiring permission to use your phone and/or your location whereby you DENY that/those permissions, the app will purposely shutdown; which begs the question: Why ask for permission in the first place?