In the opening chapter of Max Adams’ book The First Kingdom he mentions that many villas in Roman Britain were owned by Gallo-Roman civil servants and that they were still visited and used after the withdrawal. I also remember seeing archaeological analysis of a mosaic found in Gloucestershire that indicated it was laid down in 430 AD.
Apologies if this question is too broad. I’m just interested in how some cities became abandoned and gained little significance after (such as Letocetum) and some survived to this day in some recognisable stature (such as Camulodunum).
The first thing to say is that the Roman military withdrawal from Britain is not well-understood. We have a thirty year period in which we know it happened and a somewhat blurry end-date (411) when the Emperor Honorius reportedly wrote that Britain should "look to its own defence". It's clear from both the historical and archaeological record that "Roman Britain" continued for some time after - Roman towns are not abandoned and Christianity still has a significant cultural presence.
The use of buildings changed over time. The general decline of the western Roman empire had economic implications for Britain and we see infrastructure being abandoned, repurposed and run down. Roads are used but not maintained, amphitheatres become market places, public buildings might become stables. In the case of large villas, there is a general retreat from outer buildings (which then are used for storage or animal husbandry) to a new 'core' that could be easily heated. Churches tend to go through several types of use before becoming churches again in later centuries.
How life changed varies a lot by region but we don't always have much archaeology to work with - there has historically been a lack of interest in the (long) period between Roman and Saxon hegemony. We know that settlements like Tintagel on the south west coast maintained trade links to the Mediterranean for a long time after 411. By contrast the south east experienced Saxon migration quite early on. Thomas Williams' Lost Realms was published this year (2022) and is up-front about what we do and don't know about Britain in the Early Medieval.