If i have a holy symbol as a paladin or cleric, can i cast most spells? Same goes with arcane focus and component pouch. Would i eventually need to get the materials? i know for higher spells that require specific items but there are some low spells that require material too. Do spells need those material as long as i have the holy symbol/arcane focus/component pouch? This has probably been answered before but new to this.
They replace material components for any spells that aren't given a specific cost and are not consumed by the spell. So, if you have a low level spell such as the cantrip Minor Illusion that uses a material component (a bit of fleece), having a component pouch/arcane focus would replace the need to have that component. However, the spell Find Familiar that uses both a component with a specific cost AND a component that is consumed by the spell would not be able to use these objects.
In short, no difference. Some games like to track material components, but even then a component pouch theoretically 'replaces' the non-costed components. If a spell says you need bat guano or three drops of water or whatever, then having either a focus or a pouch fulfils that need. If the spell says you need a 100gp pearl or it consumes incense worth 200gp, then you need to actually have those things (in my games I abstract those things into just having gold enough to cast the spell). Beyond that a focus and a pouch are identical. The only difference with a holy symbol is that it can include a holy symbol or emblem emblazoned on a shield - turning a shield into an effective focus for clerics and paladins.
It is worth noting that (RAW) when holding such a holy shield or focus object, you are able to cast any spell with Material components: V, M, VM or VSM - but you can't cast spells with only Somatic components: S or VS. Thus a shield-holding cleric can't cast Cure Wounds without putting their weapon away first.
All other differences are pure colour. Personally I like a wizard to use a pouch and components and alchemy and such, but a sorcerer should use a wand or crystal to harnass their raw power.
Most things have already been mentioned. A spell casting focus replaces the material component of a spell that that doesn't have a cost or is consumed by the spell rather than having to have a component pouch. The main differences between the spell casting focuses is the face that clerics and paladins can use a holy symbol focus on their shield and still have it count as them manipulating the material component, where as a wizard or eldrich knight can't stick a crystal ball on a shield and have it work the same way. Same as if a cleric lost there shield with their holy symbol on it they can't borrow the wizards arcne focus to cast spells.
Generally it all just comes down to flavor and how you and your group like to roleplay. The rules are written very well in this regard to the point it is almost unnecessary to keep track of it. The only time it might come into play is if you are say a cleric or eldrich knight and had to stow your weapon so you could cast a spell on your turn. Because you have used your free item interaction to stow your weapon, you can't draw it again until your next turn. Which basically mean should an enemy provoke an opportunity attack from you, you will not have a weapon ready to strike with. Although this last point kind of strays from the original question a little. Hope this clears things up for you.
If i have a holy symbol as a paladin or cleric, can i cast most spells? Same goes with arcane focus and component pouch. Would i eventually need to get the materials? i know for higher spells that require specific items but there are some low spells that require material too. Do spells need those material as long as i have the holy symbol/arcane focus/component pouch? This has probably been answered before but new to this.
They replace material components for any spells that aren't given a specific cost and are not consumed by the spell. So, if you have a low level spell such as the cantrip Minor Illusion that uses a material component (a bit of fleece), having a component pouch/arcane focus would replace the need to have that component. However, the spell Find Familiar that uses both a component with a specific cost AND a component that is consumed by the spell would not be able to use these objects.
In short, no difference. Some games like to track material components, but even then a component pouch theoretically 'replaces' the non-costed components. If a spell says you need bat guano or three drops of water or whatever, then having either a focus or a pouch fulfils that need. If the spell says you need a 100gp pearl or it consumes incense worth 200gp, then you need to actually have those things (in my games I abstract those things into just having gold enough to cast the spell). Beyond that a focus and a pouch are identical. The only difference with a holy symbol is that it can include a holy symbol or emblem emblazoned on a shield - turning a shield into an effective focus for clerics and paladins.
It is worth noting that (RAW) when holding such a holy shield or focus object, you are able to cast any spell with Material components: V, M, VM or VSM - but you can't cast spells with only Somatic components: S or VS. Thus a shield-holding cleric can't cast Cure Wounds without putting their weapon away first.
All other differences are pure colour. Personally I like a wizard to use a pouch and components and alchemy and such, but a sorcerer should use a wand or crystal to harnass their raw power.
Most things have already been mentioned. A spell casting focus replaces the material component of a spell that that doesn't have a cost or is consumed by the spell rather than having to have a component pouch. The main differences between the spell casting focuses is the face that clerics and paladins can use a holy symbol focus on their shield and still have it count as them manipulating the material component, where as a wizard or eldrich knight can't stick a crystal ball on a shield and have it work the same way. Same as if a cleric lost there shield with their holy symbol on it they can't borrow the wizards arcne focus to cast spells.
Generally it all just comes down to flavor and how you and your group like to roleplay. The rules are written very well in this regard to the point it is almost unnecessary to keep track of it. The only time it might come into play is if you are say a cleric or eldrich knight and had to stow your weapon so you could cast a spell on your turn. Because you have used your free item interaction to stow your weapon, you can't draw it again until your next turn. Which basically mean should an enemy provoke an opportunity attack from you, you will not have a weapon ready to strike with. Although this last point kind of strays from the original question a little. Hope this clears things up for you.
Thanks, those were all good answers and mostly how I felt about them too. Wanted other perspective.