Big and small Ideas / initiatives go through the portfolio level where these get vetted. Once approved, then it gets prioritized. Flow of work results from approved ideas or initiatives.
However, a flow of work could also enter via the solution level, program level or team level … these work are as a result of refactor, retrofit, technical debt, feedback loop work (i.e. removal of or addition of features to the product or the MVP that was released earlier in production), enablers, capabilities, spikes, etc.
Note that work could eventually hit Agile Teams, work entering from any SAFe level (Portfolio; Solutions; Program; Team) — we bring work to the people! However, also note that each Agile Team’s capacity is fixed hence prioritized work — using WSJF as a tool for prioritization — is paramount. 
Therefore, since Product Managers and Product Owners steer the ART and team respectively, they must make sure that the sequence of features and stories are correct vis-a-vis team capacity and hard milestones for value delivery. First things first, regardless of where the work is entering from.
