Bottom
Brackets
All bikes have bottom brackets, including road bikes, e-bikes, mountain bikes, hybrid bikes and gravel bikes. The bottom bracket of the bike connects the crankset of the bike to the frame - meaning that the crankset can rotate, without the entire frame rotating!
Bottom brackets will typically last for thousands of miles, which can be a couple of years for riders - although bottom brackets on mountain bikes may need replacing sooner, with ingress of mud & grime from the trails - which can be negated by a strict cleaning regimen. The biggest giveaway to a bottom bracket that needs replacing, is a wobbling crank or excessive noise from the bottom bracket.
In some cases, a noisy bottom bracket may not need replacing - it may just need the bearings replacing or greasing.