Bodhisattvas can be Buddhas. Arhats are also Buddhas, a technical class of Buddha called a Śrāvakabuddha (Sāvakabuddha in Pali). Different Mahayana teachers have different places where they assign the degree of wisdom and liberation the Arhat enjoys relative to the bodhisattva path. See this thread: https://dharmawheel.net/viewtopic.php?f=39&t=23667
Relating to that bodhisattvas can be Buddhas:
(Pañcaviṁśatisāhasrikāprajñāpāramitāsūtra quoted in Mahāprajñāpāramitopadeśa)daśamyāṃ punaḥ subhūte bhūmau vartamāno bodhisattvas tathāgata eveti vaktavyaḥ || tatra kathaṃ bodhisattvo daśamyāṃ bhūmau sthitaḥ saṃs tathāgata eveti vaktavyaḥ | yadā bodhisattvasya mahāsattvasya ṣaṭpāramitāḥ paripūrṇā bhavanti | catvāri smṛtyupasthānāni yāvad aṣṭādaśāveṇikā buddhadharmāḥ paripūrṇā bhavati | sarākārajñatā paripūrṇā bhavati | sarvakleśānāṃ tadvāsanānāṃ ca prahāṇaṃ bhavati | evaṃ hi bodhisattvo mahasattvo daśamyāṃ bhūmau sthitas tathāgata eveti vaktavyaḥ ||
Furthermore, O Subhūti, the bodhisattva on the tenth ground (dharmameghābhūmi) should simply be called "Tathāgata." Why should the bodhisattva on the tenth ground be called simply "Tathāgata?" When he completely fulfills the six perfections, when he completely fulfills the four foundations of mindfulness up to and including the eighteen special attributes of the Buddhas, when he completely fulfills the knowledge of all the aspects and when he destroys all the passions and their traces, the bodhisattva mahāsattva in the tenth ground should be called simply "Tathāgata."
Complete Buddhatva is a bhumi higher still in most interpretations of the 10 bhumis.
In addition, there is the story of Saddharmapradīpa Buddha, whose vow led him to become the bodhisattva Avalokiteśvara. See here: https://dharmawheel.net/viewtopic.php?p=530264#p530264 (this does not seem to be a common story about Avalokiteśvara at all though).