Every plant needs light. Some need more than others and how much depends on how - and where - that particular tree grows naturally. Bonsai that naturally thrive in full sun like mini jades, bougainvillea and many other flowering bonsai, need more light than more shade tolerant varieties like ficus and schefflera.
Variegated leaves and leaves with a fuzzy or waxy coating are often indicators of a need for more light than others of their kind. So a variegated Serissa is happy with more light than its solid green sibling.
Most bonsai that are grown indoors require as much light as you can give them, without keeping them so close to a southern or western window that they will burn if you don’t keep an eye on them. That happens because the window will act as a magnifying glass, greatly intensifying the light. Plants are most vulnerable to this burn when you first move them to a window position or even as the seasons change. As the sun climbs higher in the sky the light becomes stronger, so come March and April watch your trees for signs of yellowed, burnt looking or dried out leaves.
You can also look for signs that there isn’t enough light. The most obvious and first to show will be “leggy growth”, which means that there is too much space on the stems between the sets of leaves. Your tree will look like it’s reaching for the sun and if the growth is not only leggy but also noticeably stretching in one direction and that direction happens to be towards the window, it is not getting enough light but you also need to turn it around regularly to evenly distribute the light it receives.
INSIDE/OUTSIDEFirst, please understand, there is really no such thing as an indoor bonsai or indoor plants. Every plant, every bonsai, is naturally supposed to grow outside. An Indoor bonsai is usually a tree that will not survive our colder winters but will tolerate the lowered light and drier air inside our homes over the winter.
As the weather warms up, moving your trees outside is about the best thing you can do for their long term health and the easiest way for you to burn them. Trust me on this one, I’ve burnt quite a few myself.
When you first put your plants outside - put them in the shade. Don’t let any direct light hit them for a few days. Then let them get about 30 minutes of (preferably) early morning sun - for about a week. Watch the leaves for any signs of burning. After about a week you can give them more morning sun until they take full morning sun, maybe even midday depending on the tree.
A few will take mid-afternoon and early evening sun in the summer, but it really isn’t necessary.
They will need more water once you bring them outside and at summer’s end, well before any signs of frost, move them back into full shade to mitigate the shock of coming back inside.
Most bonsai will benefit from artificial grow lights. Here’s a good article about choosing indoor lighting: https://www.bonsaitreegardener.net/care/how-to/artificial-light