The prayer hall follows the ‘reverse T’ ground plan, with a central space (on the foreground of this picture), flanked by three iwan-rooms: the ‘mihrab’ area in the South (in the background) and two aisles (East and West; not visible on this picture).
The arch that separates the central space from the mihrab area is called ‘Bursa arch’; this mosque was the very first one in which this peculiar form of arch was used.
The small door in the left down corner of the picture, leads to a room in the southeast corner of the building. This room was part of the ‘zaviye’ (dervish lodge), which was integrated in this so-called ‘multi-functional mosque’; the three other ‘zaviye’-rooms are situated in the three other corners of the building.

Regarding ‘iwan’:
An iwan (Persian & Turkish: ‘eyvān’) is a rectangular hall or space, usually vaulted, walled on three sides, with one end entirely open. This architectural form can be used for entrances, but also (as it is the case here) to create half open rooms looking out at a central yard. The iwan as used in Anatolian Seljuk ‘medrese’ was imported from Islamic Persia, but was invented much earlier and fully developed in Mesopotamia.

Regarding early-Ottoman ‘multi-functional mosques’ .
The T-form ground plan of the congregation hall of early-Ottoman mosques (the so-called ‘Bursa-type’ mosques) is derived from the late-Seljuk medrese-with-covered-courtyard, with three large iwans (half-open rooms) opening upon a central space. In the older Bursa mosques, these four spaces are still distinct from each other: their floors are not on the same level.
As the first Ottoman rulers (at least up to their conquest of Constantinople) had strong ties with various Sufi Orders, it is clear that the first Bursa-type mosques were designed as multi-functional buildings to meet the needs of these brotherhoods: a place to worship (the ‘mihrab’-room), but also to teach and debate (the ‘iwan’-rooms on the left and right) and even to stay overnight (in general two or four rooms, in the corners of the building). The central space of the prayer hall acts as a ‘public square’, connecting all areas with each other.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Türkiye Tarihi Yerler Kılavuzu’ – M.Orhan Bayrak, Inkılâp Kitabevi, Istanbul, 1994.
‘Encyclopedia of the Ottoman Empire’ - Gábor Ágoston & ‎Bruce Alan Masters, 2009, Wikipedia
& ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983 .